【leetcode】Swap Nodes in Pairs

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.

 

/**
 * 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)
            return NULL;
       if (head -> next == NULL)
            return head;
       ListNode *tmp = head -> next;
       head -> next = swapPairs(tmp -> next);
       tmp -> next = head;
       return tmp;
    }
};