# Validate Binary Search Tree

## Problem

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.

## Example

An example:

      2
/ \
1   4
/ \
3   5

The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).

## Code - Java

/**
* Definition of TreeNode:
* public class TreeNode {
*    public int val;
*    public TreeNode left, right;
*    public TreeNode(int val) {
*        this.val = val;
*        this.left = this.right = null;
*     }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: True if the binary tree is BST, or false
*/
public boolean isValidBST(TreeNode root) {
return isValidBST(root, Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY);
}

public boolean isValidBST(TreeNode root, double min, double max) {
if (root == null) return true;

if (root.val >= max || root.val <= min) {
return false;
}
return isValidBST(root.left, min, root.val) && isValidBST(root.right, root.val, max);
}
}