# Validate Binary Search Tree

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

## Example

**Example 1:**

```
Input:

    2
   / \
  1   3

Output:
 true
```

**Example 2:**

```
    5
   / \
  1   4
     / \
    3   6

Output:
 false

Explanation:
 The input is: [5,1,4,null,null,3,6]. The root node's value is 5 but its right child's value is 4.
```

## Note

中序遍历或者分治（设置最大和最小的边界，根据现有的值的情况更新）

## Code

```java
class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        return helper(root, null, null);
    }

    public boolean helper(TreeNode root, Integer min, Integer max){
        if (root == null) return true;
        if (min != null && root.val <= min) return false;
        if (max != null && root.val >= max) return false;
        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
    }

    public boolean isValidBST2(TreeNode root) { //acsending order for inorder BST
       if (root == null) return true;
       Stack<TreeNode> stack = new Stack<>();
       TreeNode pre = null;
       while (root != null || !stack.isEmpty()) {
          while (root != null) {
             stack.push(root);
             root = root.left;
          }
          root = stack.pop();
          if (pre != null && root.val <= pre.val) return false; // pre is the one before root is current one 
          pre = root;
          root = root.right;
       }
       return true;
    }

}
```

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# 
class Solution:
    def isValidBST(self, root: 'TreeNode') -> 'bool':
        def helper(root, lo=float('-inf'), hi=float('inf')):
            if not root:
                return True

            if not lo < root.val < hi:
                return False

            return helper(root.left, lo, min(root.val, hi)) and \
                   helper(root.right, max(lo, root.val), hi)
        return helper(root)
```


---

# 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://luj.gitbook.io/code/tree/bst/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.