LeetCode-Validate Binary Search Tree

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isValidBST(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
//一颗二叉树是二叉搜索树的充要条件是该二叉树的中序遍历是有序的
bool ans = true;
long long minVal = -2147483649;
stack<TreeNode *> stk;
TreeNode *p = root;
while (p != NULL || !stk.empty())
{
while (p != NULL)
{
stk.push(p);
p = p->left;
}
p = stk.top();
stk.pop();
if (p->val <= minVal)
{
ans = false;
break;
}
else
{
minVal = p->val;
p = p->right;
}
}
return ans;
}
};