第24题：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 maynot modify the values in the list, only nodes itself can be changed.

题目要求：从头结点开始交换每两个相邻节点的值。

编程语言：javascript

解法：算法复杂度为O(n)  思路：注意临界条件。

/** * Definition for singly-linked list. * function ListNode(val) { *     this.val = val; *     this.next = null; * } *//** * @param {ListNode} head * @return {ListNode} */var swapPairs = function(head) {    var copyList = new ListNode(0);    copyList.next = head;            while(head !== null)    {        //两个或两个以下的情况        if( head === null || head.next === null )        {            break;        }        if(head.next.next === null)        {            var temp = head.val;            head.val = head.next.val;            head.next.val = temp;            break;        }            temp = head.val;        head.val = head.next.val;        head.next.val = temp;        head = head.next.next;                if(head.next === null) break;    }    return copyList.next;   };