Swap Nodes in Pairs Medium
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.
public ListNode swapPairs(ListNode head) { ListNode p = head; if (p == null || p.next == null) return head; ListNode newHead = p.next; p.next = p.next.next; newHead.next = p; p = newHead.next.next; newHead.next.next = swapPairs(p); return newHead; } 思路：递归，每次Swap两个，然后指向下一部分，最终返回当前部分的头给上一部分的尾。