Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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:

Input: root = [2,1,3] Output: true Example 2:

Input: root = [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.

<解題>

public class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    public boolean helper(TreeNode root, long min, long max) {
        if (root == null) return true;
        if (root.val >= max || root.val <= min) return false;
        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
    }
}

Time: O(n) Space: O(n)