LeetCode-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.

Solution:

Code:

<span style="font-size:14px;">/** * 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 *begin = head->next, *end = head, *nextNode, *nextNextNode;        head = begin->next;        begin->next = end;        while (head != NULL && head->next != NULL) {            nextNode = head;            nextNextNode = nextNode->next;            head = nextNextNode->next;            nextNextNode->next = nextNode;            end->next = nextNextNode;            end = nextNode;        }        end->next = head;        return begin;    }};</span>