leetcode 24. 两两交换链表中的节点

题目描述

代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(head==nullptr || head->next == nullptr)
            return head;
        ListNode *dummyHead = new ListNode(-1,head);
        ListNode *pre = dummyHead;
        ListNode *newHead = nullptr;
        ListNode *post;
        while(pre->next != nullptr && pre->next->next != nullptr){
            ListNode *pair1 = pre->next;
            ListNode *pair2 = pre->next->next;
            if(newHead == nullptr)
                newHead = pair2;
            post = pair2->next;
            pair2->next = pair1;
            pair1->next = post;
            pre->next = pair2;
            pre = pair1;
        }
        delete dummyHead;
        dummyHead = nullptr;
        return newHead;
    }
};