问题描述：

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keysless than the node‘s key.
• The right subtree of a node contains only nodes with keysgreater than the node‘s key.
• Both the left and right subtrees must also be binary search trees.

代码：

bool isValidBST(TreeNode *root) {  //C++
        if(root == NULL||(root->left==NULL &&root->right==NULL))
            return true;
        
        int pre;
        bool isFirst = true;
        stack<TreeNode*> myStack;
        TreeNode* p = root;
        while(p!=NULL || !myStack.empty())
        {
            while(p!=NULL){
                myStack.push(p);
                p = p->left;
            }
            p = myStack.top();
            if(isFirst){
                isFirst = false;
                pre = p->val;   
            }
            else{
                if(p->val <=pre)
                    return false;
                pre = p->val;
            }
            myStack.pop();
            p = p->right;
        }
                
        return true;
    }

[leetcode] 98 Validate Binary Search Tree