LeetCode Swap Nodes in Pairs

1.题目

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.

2.解决方案

class Solution {
public:
   
    ListNode *swapPairs(ListNode *head) {
       ListNode* returnNode  = head;

    ListNode* preNode = NULL;
    while(head != NULL && head->next != NULL){
        ListNode* firstNode = head;
        ListNode* secondNode = head->next;

        if(preNode != NULL){//first node
            preNode->next = secondNode;
        }else{
            if(secondNode != NULL){
                returnNode = secondNode;
            }
        }
        firstNode->next = secondNode->next;
        secondNode->next = firstNode;


        preNode = head;
        head = head->next;
    }

    return returnNode;

    }
};

思路:这题还是比较简单的。while循环，交换两个指针而已。但要注意细节以及返回的值。

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