LeetCode Recover Binary Search Tree

// 308ms
 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void recoverTree(TreeNode *root) {
13         // Start typing your C/C++ solution below
14         // DO NOT write int main() function
15         TreeNode *p=NULL,*q=NULL,*r,*first,*second;
16         int flag=0;
17         stack<TreeNode*> s;
18     
19         r=root;
20         while(r||!s.empty())
21         {
22             if (r)
23             {
24                 s.push(r);
25                 r=r->left;
26             }
27             else
28             {
29                 TreeNode *top=s.top();
30                 s.pop();
31                 if(p==NULL)
32                     p=top;
33                 else
34                 {
35                     q=p;
36                     p=top;
37                     if(q->val>p->val)
38                     {
39                         if(flag==0)
40                         {
41                             first=q;
42                             second=p;
43                             flag=1;
44                         }
45                         else
46                         {
47                             second=p;
48                         }
49                     }
50                 }
51                 r=top->right;
52                 
53             }
54         }
55         int t=first->val;
56         first->val=second->val;
57         second->val=t;
58     }
59 };