Update readme.md

Author: satriii

Graphs/DFS/readme.md

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-# Hey
+## Overview:
+Two techniques are frequently used for graph traversal: These techniques are called **depth-first search (DFS)** and **breadth-first search (BFS)**, although we will just talk about DFS. DFS involves diving deep into the graph and then backtrack when it reaches the bottom.
+
+## What is Depth First Search in Python?
+In DFS, *we continue to traverse downwards through linked nodes until we reach the end*, then retrace our steps to check which connected nodes we haven't visited and repeat the process. In depth-first search, we dive deep into the graph and then backtrack when we reach the bottom.
+
+## Algorithm of DFS in Python
+Every time we reach a new node, we will take the following steps:
+1. We add the node to the top of the stack.
+2. Marked it as visited.
+3. We check if this node has any adjacent nodes:
+    1. If it has adjacent nodes, then we ensure that they have not been visited already, and then visited it.
+    2. We removed it from the stack if it had no adjacent nodes.
+
+## Time & Space Complexity
+* **Time Complexity:**
+Time complexity of DFS is `O(V+|E|)`, where V is the number of vertices and E is the number of edges.
+* **Space Complexity:**
+The space complexity of the DFS algorithm is `O(V)`, where V is the number of vertices.
+
+## Input & Output:
+### Input:
+
+```python
+graph = {
+        0: [2],
+        1: [2, 3],
+        2: [0, 1, 4],
+        3: [1, 4],
+        4: [2, 3]
+    }
+ ```
+ 
+ ### Output:
+ ```python
+ Depth-first search: [0, 2, 1, 3, 4]
+ ```