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.

// Source : https://oj.leetcode.com/problems/swap-nodes-in-pairs/
// Author : Chao Zeng
// Date   : 2014-12-21
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x),next(NULL) {}
};

class Solution
{
public:
    ListNode *swapPairs(ListNode *head){
        if (!head)
            return NULL;
        ListNode *phead = head;
        int length = 0;
        while (phead){
            phead = phead->next;
            length++;
        }
        if (length == 1)
            return head;
        ListNode *first = head;
        ListNode *second = head->next;
        while (first && second){
            int elem = first->val;
            first->val = second->val;
            second->val = elem;
            first = second->next;
            if (first != NULL)
                second = first->next;
            else
                break;
        }
        return head;
    }
};