https://leetcode-cn.com/problems/swap-nodes-in-pairs/
每两个节点为一组，进行遍历，
// 1. prev - a(head) - b - c
// 2. prev - a = b c
// prev
// |
// 3. b - a - c
// 4. prev - b - a - c(head)
见 index.js
https://leetcode-cn.com/problems/swap-nodes-in-pairs/
每两个节点为一组，进行遍历，
// 1. prev - a(head) - b - c
// 2. prev - a = b c
// prev
// |
// 3. b - a - c
// 4. prev - b - a - c(head)
见 index.js