24. 两两交换链表中的节点

[wengzc]

给定一个链表，两两交换其中相邻的节点，并返回交换后的链表。

你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

 

示例:

给定 1->2->3->4, 你应该返回 2->1->4->3.

题目链接：https://leetcode-cn.com/problems/swap-nodes-in-pairs

[wengzc]

解法一：递归法

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function (head) {
    if (!head || !head.next) return head
    let next = head.next
    head.next = swapPairs(next.next)
    next.next = head
    return next
};

解法二：迭代法

var swapPairs = function (head) {
    let dummy = new ListNode()
    dummy.next = head
    let cur = dummy
    while (head && head.next) {
        let first = head, second = head.next
        first.next = second.next
        second.next = first
        cur.next = second
        cur = first
        head = first.next
    }
    return dummy.next
};