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.

思路：

基本的链表操作题，用迭代方法实现，首先建立dummy节点，最后注意转换相邻链表后的链表连接。

var swapPairs = function(head) {
    var dummy=new ListNode(-1);
    var pre=dummy;
    dummy.next=head;
    while(pre.next && pre.next.next){
        var t=pre.next.next;
        pre.next.next=t.next;
        t.next=pre.next;
        pre.next=t;
        pre=t.next;
    }
    return dummy.next; 
};