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;
    }
}