Leetcode 验证bst

class Solution {
public:
    bool isValidBST(TreeNode *root) 
    {
        stack<TreeNode *>s;
        TreeNode *p=root;
        int pre=INT_MIN;
        while(p||!s.empty())
        {
            while(p)
            {
                s.push(p);
                p=p->left;
            }
            if(!s.empty())
            {
                p=s.top();
                s.pop();
                if(p->val<=pre)return false;
                pre=p->val;
                p=p->right;
            }
        }
        return true;        
    }
};