LeetCode--swap-nodes-in-pairs

题目描述

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given1->2->3->4, you should return the list as2->1->4->3.

Your algorithm should use only constant space. You may notmodify 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 == nullptr || head->next == nullptr)
            return head;
        
        ListNode* fake = new ListNode(0);
        fake->next = head;
        ListNode* p = fake;

        
        while(p->next && p->next->next)
        {
            p->next = swap(p->next, p->next->next);
            p = p->next->next;
        }
        return fake->next;
    }
    
    ListNode* swap(ListNode* head1, ListNode* head2)
    {
        head1->next = head2->next;
        head2->next = head1;
        return head2;
    }
};