leetcode hot 25

解题思路：这题就是正常模拟，我用三个指针，left，mid，right，从dummy_head开始，一直交换即可。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null || head.next==null) return head;
        ListNode dummy_head = new ListNode(0,head);
        ListNode left = dummy_head;
        ListNode mid = head;
        ListNode right = head.next;
        while(true){
            left.next = right;
            mid.next = right.next;
            right.next = mid;
            if(mid.next==null||mid.next.next==null) break;
            left = mid;
            right = mid.next.next;
            mid = mid.next;
        }
        return dummy_head.next;
    }
}