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} head
10  * @return {ListNode}
11  */
12 var swapPairs = function(head) {
13     
14     //如果全部交換還簡單點，但是這裏要求只是相鄰的旋轉。
15     
16
 
     
17     //假設只有1個節點，那直接返回。因為後面的一個是null。
18     //假設有2個節點，那只要把second的next指向first即可，然後返回last
19     //假設有3個節點，前2個節點如第二步一樣，剩下1個節點和第一步一樣。
20     //假設有4個節點，遞歸走第二步
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 };

24. Swap Nodes in Pairs