98. 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

Recursion

```java

public boolean isValidBST(TreeNode root) { return check(root, null, null); }

public boolean check(TreeNode root, Integer min, Integer max) { // if current node valid if(root == null) return true; if(min != null && root.val <= min) return false; if(max != null && root.val >= max) return false; // if children valid return check(root.left, min, root.val) && check(root.right, root.val, max); }```