Decode String (Non-forbidden Way)

class Solution {
public:
    string decodeString(string s) {
        stack<string> strings;
        stack<int> multipliers;

        string ans = "";
        for (int i = 0; i < (int) s.size(); i++) {
            if (s[i] == '[') {
                strings.push("");
            } else if (letter(s[i])) {
                if (strings.empty()) ans += s[i];
                else strings.top() += s[i];
            } else if (digit(s[i])) {
                if (i == 0 || !digit(s[i - 1])) multipliers.push(s[i] - '0');
                else {
                    multipliers.top() *= 10;
                    multipliers.top() += s[i] - '0';
                }
            } else {
                int multiplier = multipliers.top();
                multipliers.pop();

                string str = strings.top();
                strings.pop();

                while (multiplier--) {
                    if (strings.empty()) ans += str;
                    else strings.top() += str;
                }
            }
        }

        return ans;
    }

private:
    bool letter(char c) {
        return c >= 'a' && c <= 'z';
    }

    bool digit(char c) {
        return c >= '0' && c <= '9';
    }
};