decompressor-stack.js

/*
 * Given an encoded string, return it's decoded string.
 *
 * Examples:
 *
 * s = "3[a]2[bc]", return "aaabcbc".
 * s = "3[a2[c]]", return "accaccacc".
 * s = "2[abc]3[cd]ef", return "abcabccdcdcdef".
 * s = "3[a]2[b4[F]c]", return "aaabFFFFcbFFFFc".
 *
 * Live example https://jsbin.com/jekodiv/4/edit?js,console
 */


const proccess = (arr) => {
    const res = [];

    while (arr.length) {
        const num = arr.shift();
        const val = arr.shift();

        if (Number(num)) {
            if (Array.isArray(val)) {
                const p = proccess(val);

                res.push(p.repeat(num || 1));
            } else {
                res.push(val.repeat(num || 1));
            }
        } else {
            res.push(val);
        }
    }

    return res.join("");
};

function parse(stack) {
    let res = [];
    let curNum = [];

    while (stack.length) {
        let c = stack.shift();

        if (!isNaN(Number(c))) {
            curNum.push(c);
        } else if (c === "[") {
            const s2 = parse(stack);
            res.push(Number(curNum.join("")));
            res.push(s2);
            curNum = [];
        } else if (c !== "]") {
            res.push(0);
            res.push(c);
        } else if (c === "]") {
            return res;
        }
    }

    return res;
}


export const decodeString = function (s) {
    return proccess(parse(s.split("")));
};