/*
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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode *dummy = new ListNode(0);
dummy->next = head;
ListNode* h = dummy;
ListNode *tmp1 = NULL, *tmp2 = NULL, *tmp3 = NULL;
while (h->next && h->next->next)
{
tmp1 = h->next;
tmp2 = h->next->next;
tmp3 = h->next->next->next;
h->next = tmp2;
tmp2->next = tmp1;
tmp1->next = tmp3;
h = tmp1;
}
ListNode* res= dummy->next;
delete dummy;
return res;
}
};

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