### leetcode Question 108: Swap Nodes in Pairs

Swap Nodes in Pairs

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.

Analysis:
This is just a pointer problem which is not that hard to think and implement.
We deal only when the next two nodes exist, e.g. if there are 3 nodes, we only take care of the first 2.
As required only 2 extra ListNodes is needed (definitely you can use only one), to store the current 2 nodes. Then just swap them and link them.

The code is pretty clear , see below for details.

Code:
/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode *p = new ListNode(0);
while(true){
if (p->next==NULL){break;}
if (p->next->next==NULL){break;}
ListNode* q1 = p->next;
ListNode* q2 = q1->next;
q1->next = q2->next;
q2->next = q1;
p->next = q2;
p=q1;
}