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.
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 ListNode *swapPairs(ListNode *head) { 12 if (head == NULL) 13 return NULL; 14 if (head -> next == NULL) 15 return head; 16 ListNode *tmp = head -> next; 17 head -> next = swapPairs(tmp -> next); 18 tmp -> next = head; 19 return tmp; 20 } 21 };
原文：http://www.cnblogs.com/jawiezhu/p/4452970.html