# 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.

#### Example 1:

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

Input: [2,1,3] Output: true

#### Example 2:

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

Input: [5,1,4,null,null,3,6] Output: false

Explanation: The root node's value is 5 but its right child's value is 4.

``````class Solution {
public boolean isValidBST(TreeNode root) {
return isValid(root, null, null);
}

private boolean isValid(TreeNode node, Integer min, Integer max) {
if(node == null) {
return true;
}

if((min != null && node.val <= min) ||
(max != null && node.val >= max)) {
return false;
}

return isValid(node.left, min, node.val)
&& isValid(node.right, node.val, max);
}
}``````