Number of Atoms — C++

// Time:  O(n)
// Space: O(n)

class Solution {
public:
    string countOfAtoms(string formula) {
        stack<map<string, int>> stk;
        stk.emplace();
        int submatches[] = { 1, 2, 3, 4, 5 };
        const auto e = regex("([A-Z][a-z]*)(\\d*)|(\\()|(\\))(\\d*)");
        for (regex_token_iterator<string::iterator> it(formula.begin(), formula.end(), e, submatches), end;
             it != end;) {
            const auto& name = (it++)->str();
            const auto& m1 = (it++)->str();
            const auto& left_open = (it++)->str();
            const auto& right_open = (it++)->str();
            const auto& m2 = (it++)->str();
            if (!name.empty()) {
                stk.top()[name] += stoi(!m1.empty() ? m1 : "1");
            }
            if (!left_open.empty()) {
                stk.emplace();
            }
            if (!right_open.empty()) {
                const auto top = move(stk.top()); stk.pop();
                for (const auto& kvp: top) {
                    stk.top()[kvp.first] += kvp.second * stoi(!m2.empty() ? m2 : "1");
                }
            }
        }
        string result;
        for (const auto& kvp : stk.top()) {
            result += kvp.first;
            if (kvp.second > 1) {
                result += to_string(kvp.second);
            }
        }
        return result;
    }
};