24. 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) {        ListNode *p=head;    int temp;        if(p==NULL) return head;    while(p->next)    {    temp=p->next->val;    p->next->val=p->val;    p->val=temp;    if(p->next->next)     p=p->next->next;    else break;    }    return head;    }};