valid_bst.js

const sam1 = [3, 2, 1, 5, 4, 6]
const sam2 = [1, 2, 3]
const sam3 = [1, 3, 4, 2]
const sam4 = [3, 4, 5, 1, 2]

function Node(data) {
    this.data = data;
    this.left = null;
    this.right = null;
}

function BinarySearchTree() {
    this.insert = function(root, data, p_node = null) {
        if (root === null) {
            this.data = new Node(data);
            return this.data;
        }

                if (p_node)
                    console.log(p_node.data, root.data, data)

        if (data <= root.data) {
            if (root.left) {
                this.insert(root.left, data, root);
            } else if (root.right == null) {
                root.left = new Node(data);
            }
        } else {
            if (root.right) {
                this.insert(root.right, data, root);
            } else {
                root.right = new Node(data);
            }
        }

        return this.root;
    };
}

function getSize(root) {
    if (root == null)
        return 0
    return getSize(root.left) + getSize(root.right) + 1
}




function solution(arr) {
    // generate node
    const root = new BinarySearchTree().insert(null, arr.slice(0, 1)[0]);
    arr.slice(1, arr.length).forEach(it => new BinarySearchTree().insert(root, it));

    return getSize(root) == arr.length
}

console.log(`expect\t: true \nactual\t: ` +  solution(sam1))
console.log(`expect\t: true \nactual\t: ` +  solution(sam2))
console.log(`expect\t: false \nactual\t: ` +  solution(sam3))
console.log(`expect\t: false \nactual\t: ` +  solution(sam4))