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:

/** * 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 || !head->next) return head;        ListNode* lead = new ListNode(-1);        lead->next = head;        ListNode *p = lead, *tmp;        while(head && head->next)        {            tmp = head->next;            head->next = head->next->next;            tmp->next = head;            p->next = tmp;            head = head->next;            p = p->next->next;        }        return lead->next;    }};