Comparing Left and Right Branch of a Complete Binary Tree

index.js

const solution = (arr) => {
    if (!arr) return "";
    if (arr.length === 0) return "";    
    const sum = (arr, idx) => {
        if (idx - 1 < arr.length) {
            if (arr[idx - 1] === -1) return 0;
            return arr[idx - 1] + sum(arr, idx * 2) + sum(arr, idx * 2 + 1);
        }
        return 0;
    };
    const left = sum(arr, 2);
    const right = sum(arr, 3);
    return (left == right) ? "" : (left > right ? "Left" : "Right");
};