24. Swap Nodes in Pairs

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.

原題鏈接：https://leetcode.com/problems/swap-nodes-in-pairs/

思路
需要一個輔助指針來幫忙。這裏就是指針指向的問題，畫圖即可

代碼（C++）

/**
 * 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==NULL || head->next==NULL) 
            return head;
        ListNode *helper=new ListNode(0);
        ListNode *ret=head;
        ListNode *cur=helper;
        while(ret  && ret->next)
        {
            ListNode *next=ret->next->next;
            cur->next=ret->next;
            cur=cur->next;
            cur->next=ret;
            cur=cur->next;
            cur->next=NULL;
            ret=next;
        }
        if(ret) 
            cur->next=ret;
        return helper->next;
    }
};