checkValidBST.java

import java.util.ArrayList;
import java.util.List;

public class ValidBST {

    public static class Node {
        Node left;
        Node right;
        int x;
    }

    public static void main(String[] args) {
        Node a = new Node();
        isValidBst(a);
    }

    private static void createInOrderArr(List<Node> arr, Node curr) {
        if (curr == null) {
            return;
        }

        createInOrderArr(arr, curr.left);
        arr.add(curr);
        createInOrderArr(arr, curr.right);
    }


    private static List<Node> createInOrderArr(Node curr) {
        List<Node> arr = new ArrayList<>();

        if (curr == null) {
            return arr;
        }

        createInOrderArr(arr, curr);
        return arr;
    }


    private static boolean isValidBst(Node curr) {
        List<Node> inOrderArr = createInOrderArr(curr);
        for (int i = 1; i < inOrderArr.size(); i++) {
            if (inOrderArr.get(i - 1).x > inOrderArr.get(i).x) {
                return false;
            }
        }
        return true;
    }
}