[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

/**
 * 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 dummy(0);
        ListNode *prev = &dummy;
        prev->next = head;
        while(head && head->next)
        {
            ListNode *tmp = head->next->next;
            head->next->next = head;
            prev->next = head->next;
            head->next = tmp;
            prev = head;
            head = tmp;
        }
        return dummy.next;
    }
};