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

Author: Abe0770

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

@@ -0,0 +1,41 @@
+/*
+  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;
+ *     struct ListNode *next;
+ * };
+ */
+struct ListNode* swapPairs(struct ListNode* head) {
+    if (head == NULL || head->next == NULL) 
+        return head;
+
+    struct ListNode *new_head = head->next;
+    struct ListNode *prev = NULL;
+
+    while (head != NULL && head->next != NULL) {
+        struct ListNode *next_pair = head->next->next;
+        struct ListNode *second = head->next;
+
+        if (prev != NULL)
+            prev->next = second;
+
+        second->next = head;
+        head->next = next_pair;
+
+        prev = head;
+        head = next_pair;
+    }
+
+    return new_head;
+}