leetcode24. 两两交换链表中的节点

/**
 * 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)
            return null;
        ListNode pre = head,p = head.next;
        //初始两个节点单独处理
        if(p != null){
            pre.next = p.next;
            p.next = pre;
            head = p;
            p = pre.next;
        }
        //p指针指向两个节点较前面的一个节点，pre指向p的前一个节点
        while(p != null && p.next != null){
            pre.next = p.next;
            p.next = p.next.next;
            pre.next.next = p;
            pre = p;
            p = p.next;
        }

        return head;
    }
}