leetcode: 24. Swap Nodes in Pairs

Difficulty

Medium

Description

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Solution

2ms, 100%
Time Complexity: O(n)
Space Complexity: O(1)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode fakeHead = new ListNode(0);
		fakeHead.next = head;
		ListNode pre = fakeHead;

		while (head != null && head.next != null)
		{
			ListNode swap = head.next;
			head.next = swap.next;
			swap.next = head;
			pre.next = swap;
			pre = head;
			head = head.next;
		}

		return fakeHead.next;
    }
}