LeetCode 024 Swap Nodes in Pairs 交换链表中相邻的两个节点

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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(!head)
            return head;
        ListNode* dummy=new ListNode(-1),*pre=dummy;
        dummy->next=head;
        while(pre->next&&pre->next->next)
        {
            ListNode* last=pre->next->next;
            pre->next->next=last->next;
            last->next=pre->next;
            pre->next=last;
            pre=last->next;
        }
        return dummy->next;
    }
};