LC_24. Swap Nodes in Pairs

https://leetcode.com/problems/swap-nodes-in-pairs/description/

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.

 1 public ListNode swapPairs(ListNode head) {
 2         //如果是最后一对的话，head.next.next 是可以为 NULL 的，所以不要进行判断
 3         if (head == null || head.next == null) return head;
 4         //base
 5         ListNode newHead = swapPairs(head.next.next) ;//head is 1, newHead is 4
 6         //currently head is 1,   4->3->null
 7         ListNode tail = head.next ; //tail is 2
 8         head.next = newHead ; //1->4
 9         tail.next = head ; //2->1
10         return tail;
11     }

2-1-4-3-5

5 自己就弹了，所以没有后面的处理，所以(head=3)head.next 还是直接连 last pop newhead(5) 所以是 2-1-4-3-5

要深刻理解当时stack 里 head = 3 newhead=5 这一点！