Decode String

class Solution {
public:
    string decodeString(string s) {
        int idx = 0;
        string ans = "";

        while (idx < (int) s.size()) {
            if (s[idx] >= 'a' && s[idx] <= 'z') {
                ans += s[idx++];
            } else {
                ans += decodeString(s, idx);
            }
        }

        return ans;
    }

private:
    string decodeString(string &s, int &idx) {
        int k = 0;
        while (idx < (int) s.size() && s[idx] >= '0' && s[idx] <= '9') {
            k *= 10;
            k += s[idx++] - '0';
        }

        idx++;
        string inside = "";

        while (idx < (int) s.size() && s[idx] != ']') {
            if (s[idx] >= 'a' && s[idx] <= 'z') {
                inside += s[idx++];
            } else {
                inside += decodeString(s, idx);
            }
        }

        string ans = "";
        while (k--) {
            ans += inside;
        }

        idx++;
        return ans;
    }
};