# 98 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 1:

``````    2
/ \
1   3
``````

Binary tree [2,1,3], return true. Example 2:

``````    1
/ \
2   3
``````

Binary tree [1,2,3], return false.

### Solutions

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
return validate(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}
public static boolean validate(TreeNode root, long min, long max) {
if (root == null) {
return true;
}
if (root.val < min || root.val > max){
return false;
}
// left subtree must be < root.val && right subtree must be > root.val
return validate(root.left, min, (long)root.val - 1) && validate(root.right, (long)root.val + 1, max);
}
}
``````