Will the Guru

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        return solve(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    private boolean solve(TreeNode node, long min, long max) {
        if (node == null) {
            return true;
        } else if (node.val <= min || node.val >= max) {
            return false;
        }


        return solve(node.left, Math.min(min, node.val), node.val) && solve(node.right, node.val, Math.max(max, node.val));
    }

}