Is Subtree

let isSubtree = function(mainTree, subTree) {

    function isSame(left, right) {
        if (left === right) return true;
        if (left === null && right !== null) return false;
        if (left !== null && right === null) return false;
        if (left.val !== right.val) return false;
        return isSame(left.left, right.left)
            && isSame(left.right, right.right);
    }

    if (mainTree === subTree) return true;
    if (!mainTree) return false;
    if (isSame(mainTree, subTree)) return true;

    return isSubtree(mainTree.left, subTree) || isSubtree(mainTree.right, subTree);
};