Add LeetCode 21 solution

Author: Tahanima

Algorithms/Easy/21_MergeTwoSortedLists/Solution.java

@@ -0,0 +1,76 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        if (list1 == null) {
+            return list2;
+        }
+        
+        if (list2 == null) {
+            return list1;
+        }
+        
+        ListNode mergedHead = null;
+        ListNode temporaryHead = null;
+        
+        while (list1 != null || list2 != null) {
+            if (list1 == null && list2 != null) {
+                ListNode node = new ListNode(list2.val);
+                
+                if (mergedHead == null) {
+                    mergedHead = node;
+                } else {
+                    temporaryHead.next = node;
+                }
+                
+                temporaryHead = node;
+                list2 = list2.next;
+            } else if (list1 != null && list2 == null) {
+                ListNode node = new ListNode(list1.val);
+                
+                if (mergedHead == null) {
+                    mergedHead = node;
+                } else {
+                    temporaryHead.next = node;
+                }
+                
+                temporaryHead = node;
+                list1 = list1.next;
+            } else {
+               if (list1.val < list2.val) {
+                   ListNode node = new ListNode(list1.val);
+                   
+                   if (mergedHead == null) {
+                       mergedHead = node;
+                   } else {
+                       temporaryHead.next = node;
+                   }
+                   
+                   temporaryHead = node;
+                   list1 = list1.next;
+               } else {
+                   ListNode node = new ListNode(list2.val);
+                   
+                   if (mergedHead == null) {
+                       mergedHead = node;
+                   } else {
+                       temporaryHead.next = node;
+                   }
+                   
+                   temporaryHead = node;
+                   list2 = list2.next;
+               }
+            }
+        }
+        
+        return mergedHead;
+    }
+}