LeetCode Recover Binary Search Tree

// 308ms
 1 /**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void recoverTree(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode *p=NULL,*q=NULL,*r,*first,*second;
        int flag=0;
        stack<TreeNode*> s;
    
        r=root;
        while(r||!s.empty())
        {
            if (r)
            {
                s.push(r);
                r=r->left;
            }
            else
            {
                TreeNode *top=s.top();
                s.pop();
                if(p==NULL)
                    p=top;
                else
                {
                    q=p;
                    p=top;
                    if(q->val>p->val)
                    {
                        if(flag==0)
                        {
                            first=q;
                            second=p;
                            flag=1;
                        }
                        else
                        {
                            second=p;
                        }
                    }
                }
                r=top->right;
                
            }
        }
        int t=first->val;
        first->val=second->val;
        second->val=t;
    }
};