Create 0024-swap-nodes-in-pairs.cpp

Author: Abe0770

Diff for: cpp/0024-swap-nodes-in-pairs.cpp

@@ -0,0 +1,46 @@
+/*
+  Given a linked list, swap every two adjacent nodes and return its head. 
+  You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)
+
+  Ex. Input: head = [1,2,3,4]
+      Output: [2,1,4,3]
+
+  Time  : O(N);
+  Space : O(1);
+*/
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* swapPairs(ListNode* head) {  
+        if (!head || !head->next) 
+            return head;
+
+        ListNode *new_head = head->next;
+        ListNode *prev = NULL;
+
+        while (head && head->next) {
+            ListNode *next_pair = head->next->next;
+            ListNode *second = head->next;
+
+            if (prev)
+                prev->next = second;
+
+            second->next = head;
+            head->next = next_pair;
+
+            prev = head;
+            head = next_pair;
+        }
+        return new_head;
+    }
+};