LeetCode Swap Nodes in Pairs

Description

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.

The original problem is here.

My code is here.

My Solution

I solve this problem in C++, as below:

/*
*Swap Nodes in Pairs
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/

/**
 * 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)
            return head;
        if(head->next == NULL)
            return head;
        
        ListNode * currentNode = head->next;
        ListNode * lastNode = head;
        ListNode * tempNode;
        if(head->next != NULL)
        {
            tempNode = head;
            head = head->next;
            tempNode->next = head->next; 
            head->next = tempNode;
            lastNode = head->next;
            currentNode = lastNode->next;
        }
        while(currentNode != NULL )
        {
            if(currentNode->next == NULL)
                break;
            tempNode = currentNode;
            currentNode = currentNode->next;
            tempNode->next = currentNode->next;
            currentNode->next = tempNode;
            
            lastNode->next = currentNode;
            
            lastNode    = currentNode->next;
            currentNode = lastNode->next;
        }
        return head;
    }
};

Note

1. We must swap the nodes instead of values.
2. When swap two nodes, we must assign the next value of the last node to the current node.