Leetcode-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.

Have you met this question in a real interview?

Solution:

/**
* 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 || head.next==null) return head;
ListNode preHead = new ListNode(0);
preHead.next = head;
ListNode pre = preHead;
ListNode cur = head;

while (cur!=null && cur.next!=null){
ListNode temp = cur.next.next;
pre.next = cur.next;
cur.next.next = cur;
cur.next = temp;
pre=cur;
cur = cur.next;
}

return preHead.next;
}
}