lintcode-Swap Nodes in Pairs -451

Given a linked list, swap every two adjacent nodes and return its head.

样例

Given 1->2->3->4, you should return the list as 2->1->4->3.

挑战

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||!head->next)            return head;        ListNode *ptr=head;        while(ptr){            if(!ptr->next)                break;            swap(ptr->next->val,ptr->val);            ptr=ptr->next->next;            }        return head;    }};