24. Swap Nodes in Pairs

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.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

迭代，记得画个图，不然会懵逼

/**
 * 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 dummy = new ListNode(-1), pre = dummy;
        dummy.next = head;
        while (pre.next !=null && pre.next.next!=null) {//注意leetcode给的linked list，head已经包含了有用内容。
           ListNode swap1 = pre.next;
            ListNode swap2 = pre.next.next;
            pre.next = swap2;
            swap1.next = swap2.next;
            swap2.next = swap1;
            pre = swap1;
        }
             return dummy.next;
    }
       
}