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‎README.md‎

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225225

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| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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|-|-|-|-|-|-
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| 0121 |[Best Time to Buy and Sell Stock](src/main/php/g0101_0200/s0121_best_time_to_buy_and_sell_stock)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Big_O_Time_O(N)_Space_O(1) | 282 | 43.99
228229

229230
#### Day 8
230231

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353354

354355
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
355356
|-|-|-|-|-|-
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| 0104 |[Maximum Depth of Binary Tree](src/main/php/g0101_0200/s0104_maximum_depth_of_binary_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(H) | 9 | 63.06
356358

357359
#### Day 11 Containers and Libraries
358360

@@ -620,11 +622,13 @@
620622

621623
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
622624
|-|-|-|-|-|-
625+
| 0121 |[Best Time to Buy and Sell Stock](src/main/php/g0101_0200/s0121_best_time_to_buy_and_sell_stock)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Big_O_Time_O(N)_Space_O(1) | 282 | 43.99
623626

624627
#### Day 6 Tree
625628

626629
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
627630
|-|-|-|-|-|-
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| 0102 |[Binary Tree Level Order Traversal](src/main/php/g0101_0200/s0102_binary_tree_level_order_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(N) | 4 | 96.08
628632

629633
#### Day 7 Binary Search
630634

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756760

757761
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
758762
|-|-|-|-|-|-
763+
| 0101 |[Symmetric Tree](src/main/php/g0101_0200/s0101_symmetric_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(log(N)) | 6 | 76.40
759764

760765
#### Day 16 Design
761766

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791796

792797
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
793798
|-|-|-|-|-|-
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| 0136 |[Single Number](src/main/php/g0101_0200/s0136_single_number)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Bit_Manipulation, Big_O_Time_O(N)_Space_O(1) | 33 | 80.08
794800
| 0007 |[Reverse Integer](src/main/php/g0001_0100/s0007_reverse_integer)| Medium | Top_Interview_Questions, Math | 3 | 90.99
795801
| 0009 |[Palindrome Number](src/main/php/g0001_0100/s0009_palindrome_number)| Easy | Math | 23 | 76.73
796802

@@ -813,6 +819,7 @@
813819

814820
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
815821
|-|-|-|-|-|-
822+
| 0121 |[Best Time to Buy and Sell Stock](src/main/php/g0101_0200/s0121_best_time_to_buy_and_sell_stock)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Big_O_Time_O(N)_Space_O(1) | 282 | 43.99
816823
| 0001 |[Two Sum](src/main/php/g0001_0100/s0001_two_sum)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Hash_Table, Big_O_Time_O(n)_Space_O(n) | 9 | 97.47
817824
| 0055 |[Jump Game](src/main/php/g0001_0100/s0055_jump_game)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Greedy, Big_O_Time_O(n)_Space_O(1) | 129 | 74.56
818825
| 0075 |[Sort Colors](src/main/php/g0001_0100/s0075_sort_colors)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Sorting, Two_Pointers, Big_O_Time_O(n)_Space_O(1) | 3 | 88.78
@@ -849,6 +856,7 @@
849856

850857
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
851858
|-|-|-|-|-|-
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| 0114 |[Flatten Binary Tree to Linked List](src/main/php/g0101_0200/s0114_flatten_binary_tree_to_linked_list)| Medium | Top_100_Liked_Questions, Depth_First_Search, Tree, Binary_Tree, Stack, Linked_List, Big_O_Time_O(N)_Space_O(N) | 3 | 100.00
852860
| 0024 |[Swap Nodes in Pairs](src/main/php/g0001_0100/s0024_swap_nodes_in_pairs)| Medium | Top_100_Liked_Questions, Linked_List, Recursion, Big_O_Time_O(n)_Space_O(1) | 3 | 85.71
853861
| 0021 |[Merge Two Sorted Lists](src/main/php/g0001_0100/s0021_merge_two_sorted_lists)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Linked_List, Recursion, Big_O_Time_O(m+n)_Space_O(m+n) | 3 | 90.57
854862
| 0025 |[Reverse Nodes in k-Group](src/main/php/g0001_0100/s0025_reverse_nodes_in_k_group)| Hard | Top_100_Liked_Questions, Linked_List, Recursion, Big_O_Time_O(n)_Space_O(k) | 3 | 100.00
@@ -858,6 +866,9 @@
858866
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
859867
|-|-|-|-|-|-
860868
| 0094 |[Binary Tree Inorder Traversal](src/main/php/g0001_0100/s0094_binary_tree_inorder_traversal)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Tree, Binary_Tree, Stack, Big_O_Time_O(n)_Space_O(n) | 3 | 82.09
869+
| 0102 |[Binary Tree Level Order Traversal](src/main/php/g0101_0200/s0102_binary_tree_level_order_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(N) | 4 | 96.08
870+
| 0104 |[Maximum Depth of Binary Tree](src/main/php/g0101_0200/s0104_maximum_depth_of_binary_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(H) | 9 | 63.06
871+
| 0124 |[Binary Tree Maximum Path Sum](src/main/php/g0101_0200/s0124_binary_tree_maximum_path_sum)| Hard | Top_100_Liked_Questions, Top_Interview_Questions, Dynamic_Programming, Depth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(N) | 16 | 100.00
861872
| 0098 |[Validate Binary Search Tree](src/main/php/g0001_0100/s0098_validate_binary_search_tree)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Tree, Binary_Tree, Binary_Search_Tree, Big_O_Time_O(N)_Space_O(log(N)) | 10 | 70.97
862873

863874
#### Udemy Trie and Heap
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917928

918929
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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|-|-|-|-|-|-
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| 0121 |[Best Time to Buy and Sell Stock](src/main/php/g0101_0200/s0121_best_time_to_buy_and_sell_stock)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Big_O_Time_O(N)_Space_O(1) | 282 | 43.99
920932

921933
#### Day 4 Array
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961973

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| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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|-|-|-|-|-|-
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| 0102 |[Binary Tree Level Order Traversal](src/main/php/g0101_0200/s0102_binary_tree_level_order_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(N) | 4 | 96.08
977+
| 0104 |[Maximum Depth of Binary Tree](src/main/php/g0101_0200/s0104_maximum_depth_of_binary_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(H) | 9 | 63.06
978+
| 0101 |[Symmetric Tree](src/main/php/g0101_0200/s0101_symmetric_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Big_O_Time_O(N)_Space_O(log(N)) | 6 | 76.40
964979

965980
#### Day 12 Tree
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984999

9851000
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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|-|-|-|-|-|-
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| 0136 |[Single Number](src/main/php/g0101_0200/s0136_single_number)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Bit_Manipulation, Big_O_Time_O(N)_Space_O(1) | 33 | 80.08
9871003
| 0015 |[3Sum](src/main/php/g0001_0100/s0015_3sum)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Sorting, Two_Pointers, Big_O_Time_O(n^2)_Space_O(1), Big_O_Time_O(n\*log(n))_Space_O(n^2) | 252 | 77.94
9881004

9891005
#### Day 2 Array
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10631079

10641080
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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| 0105 |[Construct Binary Tree from Preorder and Inorder Traversal](src/main/php/g0101_0200/s0105_construct_binary_tree_from_preorder_and_inorder_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Hash_Table, Tree, Binary_Tree, Divide_and_Conquer, Big_O_Time_O(N)_Space_O(N) | 14 | 63.33
10661083

10671084
#### Day 16 Tree
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11711188

11721189
| <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- --> | <!-- -->
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| 0136 |[Single Number](src/main/php/g0101_0200/s0136_single_number)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Bit_Manipulation, Big_O_Time_O(N)_Space_O(1) | 33 | 80.08
11741192

11751193
### Algorithm II
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12981316

12991317
| # | Title | Difficulty | Tag | Time, ms | Time, %
13001318
|------|----------------|-------------|-------------|----------|--------
1319+
| 0136 |[Single Number](src/main/php/g0101_0200/s0136_single_number)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Bit_Manipulation, Data_Structure_II_Day_1_Array, Algorithm_I_Day_14_Bit_Manipulation, Udemy_Integers, Big_O_Time_O(N)_Space_O(1) | 33 | 80.08
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| 0131 |[Palindrome Partitioning](src/main/php/g0101_0200/s0131_palindrome_partitioning)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, String, Dynamic_Programming, Backtracking, Big_O_Time_O(N\*2^N)_Space_O(2^N\*N) | 159 | 61.90
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| 0128 |[Longest Consecutive Sequence](src/main/php/g0101_0200/s0128_longest_consecutive_sequence)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Hash_Table, Union_Find, Big_O_Time_O(N_log_N)_Space_O(1) | 154 | 75.61
1322+
| 0124 |[Binary Tree Maximum Path Sum](src/main/php/g0101_0200/s0124_binary_tree_maximum_path_sum)| Hard | Top_100_Liked_Questions, Top_Interview_Questions, Dynamic_Programming, Depth_First_Search, Tree, Binary_Tree, Udemy_Tree_Stack_Queue, Big_O_Time_O(N)_Space_O(N) | 16 | 100.00
1323+
| 0121 |[Best Time to Buy and Sell Stock](src/main/php/g0101_0200/s0121_best_time_to_buy_and_sell_stock)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Array, Dynamic_Programming, Data_Structure_I_Day_3_Array, Dynamic_Programming_I_Day_7, Level_1_Day_5_Greedy, Udemy_Arrays, Big_O_Time_O(N)_Space_O(1) | 282 | 43.99
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| 0114 |[Flatten Binary Tree to Linked List](src/main/php/g0101_0200/s0114_flatten_binary_tree_to_linked_list)| Medium | Top_100_Liked_Questions, Depth_First_Search, Tree, Binary_Tree, Stack, Linked_List, Udemy_Linked_List, Big_O_Time_O(N)_Space_O(N) | 3 | 100.00
1325+
| 0105 |[Construct Binary Tree from Preorder and Inorder Traversal](src/main/php/g0101_0200/s0105_construct_binary_tree_from_preorder_and_inorder_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Array, Hash_Table, Tree, Binary_Tree, Divide_and_Conquer, Data_Structure_II_Day_15_Tree, Big_O_Time_O(N)_Space_O(N) | 14 | 63.33
1326+
| 0104 |[Maximum Depth of Binary Tree](src/main/php/g0101_0200/s0104_maximum_depth_of_binary_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Data_Structure_I_Day_11_Tree, Programming_Skills_I_Day_10_Linked_List_and_Tree, Udemy_Tree_Stack_Queue, Big_O_Time_O(N)_Space_O(H) | 9 | 63.06
1327+
| 0102 |[Binary Tree Level Order Traversal](src/main/php/g0101_0200/s0102_binary_tree_level_order_traversal)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Breadth_First_Search, Tree, Binary_Tree, Data_Structure_I_Day_11_Tree, Level_1_Day_6_Tree, Udemy_Tree_Stack_Queue, Big_O_Time_O(N)_Space_O(N) | 4 | 96.08
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| 0101 |[Symmetric Tree](src/main/php/g0101_0200/s0101_symmetric_tree)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Breadth_First_Search, Tree, Binary_Tree, Data_Structure_I_Day_11_Tree, Level_2_Day_15_Tree, Big_O_Time_O(N)_Space_O(log(N)) | 6 | 76.40
13011329
| 0098 |[Validate Binary Search Tree](src/main/php/g0001_0100/s0098_validate_binary_search_tree)| Medium | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Tree, Binary_Tree, Binary_Search_Tree, Data_Structure_I_Day_14_Tree, Level_1_Day_8_Binary_Search_Tree, Udemy_Tree_Stack_Queue, Big_O_Time_O(N)_Space_O(log(N)) | 10 | 70.97
13021330
| 0096 |[Unique Binary Search Trees](src/main/php/g0001_0100/s0096_unique_binary_search_trees)| Medium | Top_100_Liked_Questions, Dynamic_Programming, Math, Tree, Binary_Tree, Binary_Search_Tree, Dynamic_Programming_I_Day_11, Big_O_Time_O(n)_Space_O(1) | 0 | 100.00
13031331
| 0094 |[Binary Tree Inorder Traversal](src/main/php/g0001_0100/s0094_binary_tree_inorder_traversal)| Easy | Top_100_Liked_Questions, Top_Interview_Questions, Depth_First_Search, Tree, Binary_Tree, Stack, Data_Structure_I_Day_10_Tree, Udemy_Tree_Stack_Queue, Big_O_Time_O(n)_Space_O(n) | 3 | 82.09

‎src/main/php/g0101_0200/s0101_symmetric_tree/readme.md‎

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[![](https://img.shields.io/github/stars/LeetCode-in-Php/LeetCode-in-Php?label=Stars&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php)
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[![](https://img.shields.io/github/forks/LeetCode-in-Php/LeetCode-in-Php?label=Fork%20me%20on%20GitHub%20&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php/fork)
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## 101\. Symmetric Tree
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Easy
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Given the `root` of a binary tree, _check whether it is a mirror of itself_ (i.e., symmetric around its center).
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**Example 1:**
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![](https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg)
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**Input:** root = [1,2,2,3,4,4,3]
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**Output:** true
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**Example 2:**
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![](https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg)
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**Input:** root = [1,2,2,null,3,null,3]
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**Output:** false
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**Constraints:**
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* The number of nodes in the tree is in the range `[1, 1000]`.
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* `-100 <= Node.val <= 100`
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**Follow up:** Could you solve it both recursively and iteratively?
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## Solution
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```php
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/**
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* Definition for a binary tree node.
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* class TreeNode {
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* public $val = null;
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* public $left = null;
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* public $right = null;
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* function __construct($val = 0, $left = null, $right = null) {
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* $this->val = $val;
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* $this->left = $left;
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* $this->right = $right;
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* }
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* }
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*/
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class Solution {
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/**
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* @param TreeNode $root
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* @return Boolean
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*/
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function isSymmetric($root) {
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if ($root == null) {
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return true;
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}
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return $this->helper($root->left, $root->right);
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}
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private function helper($leftNode, $rightNode) {
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if ($leftNode == null || $rightNode == null) {
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return $leftNode == null && $rightNode == null;
64+
}
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if ($leftNode->val != $rightNode->val) {
66+
return false;
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}
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return $this->helper($leftNode->left, $rightNode->right) && $this->helper($leftNode->right, $rightNode->left);
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}
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}
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```

‎src/main/php/g0101_0200/s0102_binary_tree_level_order_traversal/readme.md‎

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[![](https://img.shields.io/github/stars/LeetCode-in-Php/LeetCode-in-Php?label=Stars&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php)
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[![](https://img.shields.io/github/forks/LeetCode-in-Php/LeetCode-in-Php?label=Fork%20me%20on%20GitHub%20&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php/fork)
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## 102\. Binary Tree Level Order Traversal
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Medium
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Given the `root` of a binary tree, return _the level order traversal of its nodes' values_. (i.e., from left to right, level by level).
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**Example 1:**
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![](https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg)
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**Input:** root = [3,9,20,null,null,15,7]
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**Output:** [[3],[9,20],[15,7]]
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**Example 2:**
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**Input:** root = [1]
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**Output:** [[1]]
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**Example 3:**
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**Input:** root = []
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**Output:** []
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**Constraints:**
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* The number of nodes in the tree is in the range `[0, 2000]`.
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* `-1000 <= Node.val <= 1000`
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## Solution
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```php
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/**
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* Definition for a binary tree node.
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* class TreeNode {
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* public $val = null;
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* public $left = null;
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* public $right = null;
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* function __construct($val = 0, $left = null, $right = null) {
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* $this->val = $val;
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* $this->left = $left;
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* $this->right = $right;
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* }
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* }
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*/
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class Solution {
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/**
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* @param TreeNode $root
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* @return Integer[][]
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*/
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function levelOrder($root) {
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$result = array();
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if ($root == null) {
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return $result;
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}
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$queue = new \SplQueue();
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$queue->enqueue($root);
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$queue->enqueue(null);
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$level = array();
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while (!$queue->isEmpty()) {
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$root = $queue->dequeue();
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while (!$queue->isEmpty() && $root != null) {
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array_push($level, $root->val);
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if ($root->left != null) {
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$queue->enqueue($root->left);
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}
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if ($root->right != null) {
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$queue->enqueue($root->right);
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}
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$root = $queue->dequeue();
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}
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array_push($result, $level);
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$level = array();
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if (!$queue->isEmpty()) {
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$queue->enqueue(null);
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}
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}
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return $result;
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}
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}
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```

‎src/main/php/g0101_0200/s0104_maximum_depth_of_binary_tree/readme.md‎

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[![](https://img.shields.io/github/stars/LeetCode-in-Php/LeetCode-in-Php?label=Stars&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php)
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[![](https://img.shields.io/github/forks/LeetCode-in-Php/LeetCode-in-Php?label=Fork%20me%20on%20GitHub%20&style=flat-square)](https://github.com/LeetCode-in-Php/LeetCode-in-Php/fork)
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## 104\. Maximum Depth of Binary Tree
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Easy
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Given the `root` of a binary tree, return _its maximum depth_.
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A binary tree's **maximum depth** is the number of nodes along the longest path from the root node down to the farthest leaf node.
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**Example 1:**
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![](https://assets.leetcode.com/uploads/2020/11/26/tmp-tree.jpg)
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**Input:** root = [3,9,20,null,null,15,7]
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**Output:** 3
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**Example 2:**
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**Input:** root = [1,null,2]
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**Output:** 2
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**Example 3:**
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**Input:** root = []
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**Output:** 0
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**Example 4:**
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**Input:** root = [0]
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**Output:** 1
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**Constraints:**
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* The number of nodes in the tree is in the range <code>[0, 10<sup>4</sup>]</code>.
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* `-100 <= Node.val <= 100`
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## Solution
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```php
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/**
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* Definition for a binary tree node.
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* class TreeNode {
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* public $val = null;
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* public $left = null;
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* public $right = null;
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* function __construct($val = 0, $left = null, $right = null) {
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* $this->val = $val;
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* $this->left = $left;
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* $this->right = $right;
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* }
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* }
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*/
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class Solution {
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/**
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* @param TreeNode $root
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* @return Integer
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*/
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function maxDepth($root) {
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return $this->findDepth($root, 0);
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}
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private function findDepth($node, $currentDepth) {
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if ($node == null) {
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return 0;
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}
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$currentDepth++;
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return 1
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+ max($this->findDepth($node->left, $currentDepth), $this->findDepth($node->right, $currentDepth));
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}
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}
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```

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