Leetcode 24. Swap Nodes in Pairs (Easy) (cpp)

/*

24. Swap Nodes in Pairs (Easy)

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

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*/
/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head) {
if(head == NULL || head->next == NULL) return head;
ListNode* temp = head -> next;
head -> next = swapPairs(temp -> next);
temp -> next = head;
return temp;
}
};