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.
/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { * val = x; * next = null; * } * } */ public class Solution { public ListNode swapPairs(ListNode head) { if(head == null) return null; if(head.next == null) return head; ListNode first = head; ListNode second = head.next; ListNode third = second.next; second.next = first; first.next = swapPairs(third); return second; } }