Validate a Binary Search Tree

/*
 * Problem : Process a binary tree.
 * Input   : One line: the tree in level order, null for a missing child.
 * Output  : true or false.
 * Approach: Pass down a value range.
 * Time    : O(n)
 * Space   : O(h)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: A tree node has a value and links to the left and right child. null means: no child.
        static class TreeNode {
            int value;
            TreeNode left;
            TreeNode right;

            TreeNode(int value) {
                this.value = value;
            }
        }

        // This method builds a tree from the level order tokens. A queue keeps the nodes that still need children.
        static TreeNode build(String[] tokens) {
            if (tokens.length == 0 || tokens[0].equals("null")) {
                return null;
            }
            TreeNode root = new TreeNode(Integer.parseInt(tokens[0]));
            java.util.ArrayDeque<TreeNode> queue = new java.util.ArrayDeque<>();
            queue.add(root);
            int i = 1;
            while (!queue.isEmpty() && i < tokens.length) {
                TreeNode node = queue.poll();
                if (i < tokens.length && !tokens[i].equals("null")) {
                    node.left = new TreeNode(Integer.parseInt(tokens[i]));
                    queue.add(node.left);
                }
                i++;
                if (i < tokens.length && !tokens[i].equals("null")) {
                    node.right = new TreeNode(Integer.parseInt(tokens[i]));
                    queue.add(node.right);
                }
                i++;
            }
            return root;
        }

        // This method checks a subtree. All values in it must be bigger than "low" and smaller than "high".
        static boolean isValid(TreeNode node, long low, long high) {
            // Step 2: An empty subtree is valid.
            if (node == null) {
                return true;
            }

            // Step 3: The value must stay inside the allowed range. Comparing only with the parent is NOT enough.
            if (node.value <= low || node.value >= high) {
                return false;
            }

            // Step 4: The left subtree must be smaller than this node. The right subtree must be bigger.
            return isValid(node.left, low, node.value) && isValid(node.right, node.value, high);
        }

    // This method solves the task. It receives the input and returns the answer.
    static boolean solve(String s) {
        return isValid(build(s.trim().split(" ")), Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}