# 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.
```

```java
class Solution {
    public boolean isValid(TreeNode root, long min, long max) {
        if (root == null)
            return true;
        if (root.val >= min && root.val <= max) {
            return isValid(root.left, min, (long) root.val - 1) && isValid(root.right, (long) root.val + 1, max);
        }
        return false;
    }

    public boolean isValidBST(TreeNode root) {
        return isValid(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }
}
```
