CodeFights: decodeString - O(n) solution

decodeString solution.js

/**
 * Solution for this - https://codefights.com/interview-practice/task/dYCH8sdnxGf5aGkez
 */
function decodeString(s) {
    return parse(s)[0];
}

function parse(string, startIdx=0) {
    let i = startIdx;
    let res = '';
    let part = '';
    let number = '';
    
    while (i < string.length) {
        let c = string[i];
        i++;

        if (/\d/.test(c)) {
            number += c;
        } else if (c === '[') {
            [part, i] = parse(string, i);
            res += multiply(part, parseInt(number));
            number = '';
        } else if (c === ']') {
            break;
        } else {
            res += c;
        }
    }

    return [res, i];
}

function multiply(s, n) {
    let out = '';
    for (let i = 0; i < n; i++)
        out += s;
    return out;
}