Greatest Common Divisor of Strings — C++

// Time:  O(m + n)
// Space: O(1)

class Solution {
public:
    string gcdOfStrings(string str1, string str2) {
        if (str1.empty() || str2.empty()) {
            return "";
        }
        int c = gcd(str1.length(), str2.length());
        const auto& result = str1.substr(0, c);
        return check(str1, result) && check(str2, result) ? result : "";
    }

private:
    bool check(const string& s, const string& common) {
        int i = 0;
        for (const auto& c : s) {
            if (c != common[i]) {
                return false;
            }
            i = (i + 1) % common.length();
        }
        return true;
    }

    int gcd(int a, int b) {
        while (b != 0) {
            int tmp = b;
            b = a % b;
            a = tmp;
        }
        return a;
    }
};