isValidBST.js

const isValidBST = (root = [5, 1, 4, null, null, 3, 6]) => {
    for (let currentRootIndex = 0; currentRootIndex < root.length; currentRootIndex++) {
        let isValid = true;

        const rootValue = root[currentRootIndex];
        const leftChildIndex = currentRootIndex * 2 + 1;
        const rightChildIndex = currentRootIndex * 2 + 2;
        const leftChildValue = root[leftChildIndex]
        const rightChildValue = root[rightChildIndex]
        // console.log({
        //     currentRootIndex,
        //     leftChildIndex,
        //     rightChildIndex,
        //     rootValue,
        //     leftChildValue,
        //     rightChildValue
        // });

        if (leftChildValue) {
            if (leftChildValue > rootValue) return false;
        }
        if (rightChildValue) {
            if (rightChildValue < rootValue) return false;
        }
    }
    return true;
}

console.log({ isValidBST: isValidBST() });