# 98. Validate Binary Search Tree

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 keys less than the node's key.\
The right subtree of a node contains only nodes with keys greater than the node's key.\
Both the left and right subtrees must also be binary search trees.

## Thoughts

检验给定BST是否合法。BST进行中序遍历一定是升序的，利用此性质检查当前结点值是否>上个结点。

## Code

```
class Solution {
    TreeNode prev = null;
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (!isValidBST(root.left) || prev != null && prev.val >= root.val) {
            return false;
        }
        prev = root;
        return isValidBST(root.right);
    }
}
```

## Analysis

做题耗时 5min

时间复杂度同中序遍历，O(n)

## Ver.2

```
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private void pushLeft(Stack<TreeNode> stack, TreeNode node) {
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
    }

    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        pushLeft(stack, root);
        TreeNode pre = null;
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (pre != null && pre.val >= node.val) {
                return false;
            }
            pre = node;
            pushLeft(stack, node.right);
        }

        return true;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://hao-fu-1.gitbook.io/oj/binary_tree_and_divide_conquer/binary-search-tree/validate_binary_search_tree.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.