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.

/** * 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 *pList = new ListNode(0);pList->next = head;head = head->next;while (pList->next != NULL && pList->next->next != NULL) {ListNode *p1 = pList->next;pList->next = p1->next;p1->next = pList->next->next;pList->next->next = p1;pList = p1;}return head;}};