24. 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.

1 /** 2  * Definition for singly-linked list. 3  * function ListNode(val) { 4  *     this.val = val; 5  *     this.next = null; 6  * } 7  */ 8 /** 9  * @param {ListNode} head10  * @return {ListNode}11  */12 var swapPairs = function(head) {13     14     //如果全部交换还简单点，但是这里要求只是相邻的旋转。15     16     17     //假设只有1个节点，那直接返回。因为后面的一个是null。18     //假设有2个节点，那只要把second的next指向first即可，然后返回last19     //假设有3个节点，前2个节点如第二步一样，剩下1个节点和第一步一样。20     //假设有4个节点，递归走第二步21     if(head === null || head.next === null){22         return head;23     }24     25     var first = head;26     var last = head.next;27      28     first.next = swapPairs(last.next);29     last.next = first;30    31     32     return last;33     34 };