LeetCode-24.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.

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */

Solution:

1、使用前向指针

    public ListNode SwapPairs(ListNode head)
    {
            if (head == null||head.next==null)
                return head;
            ListNode current = head;
            ListNode next = current.next;
            ListNode result = next;
            ListNode pre = null;
            while (next != null && current != null)
            {
                if (pre != null)
                    pre.next = next;
                current.next = next.next;
                next.next = current;

                pre = current;
                current = current.next;
                if (current != null)
                    next = current.next;
            }
            return result;
    }

2、使用"0"指针

 public ListNode SwapPairs(ListNode head)
        {
            ListNode pre = new ListNode(0);
            pre.next = head;

            ListNode current=pre;
            ListNode first = null,sec = null;
            while (current.next != null && current.next.next != null)
            {
                first = current.next;
                sec = first.next;

                first.next = sec.next;
                sec.next = first;
                current.next = sec;
                current = sec.next;
            }
            return pre.next;
        }

3、递归

  public static ListNode SwapPairs(ListNode head)
        {
            ListNode current = head;
            if (head != null && head.next != null)
            {
                current = head.next;
                head.next = SwapPairs(current.next);
                current.next = head;
            }
            return current;
        }

效果同2