/**
 * 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) {
        if (root == null) return true;
        return dfs(root, null, null);
    }

    private boolean dfs(TreeNode root, Integer min, Integer max) {
        if (root == null) return true;

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

        return dfs(root.left, min, root.val) && dfs(root.right, root.val, max);
    }
}