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;
    }
};