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. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */ public class Solution { public ListNode swapPairs(ListNode head) { if(head == null || head.next == null) return head; ListNode ret = head.next; ListNode first = head; ListNode second = head.next; while(second.next != null) { ListNode third = second.next; second.next = first; if(third.next == null) { first.next = third; return ret; } first.next = third.next; first = third; second = third.next; } if(second.next == null) { second.next = first; first.next = null; } return ret; } }
原文：http://www.cnblogs.com/neweracoding/p/5414934.html