using System;

​

public class Solution {

    private static int FindGCD(int a, int b) {

        while (b != 0) {

            int temp = b;

            b = a % b;

            a = temp;

        }

​

        return a;

    }

    // According to question s = t + t + t + ... + t + t, means for a valid case,  str1 = nt & str2 = mt

    // So, (m + n)t = (n + m)t && GCD of n & m is the GCD string

    // Example 1 # "ABABAB" & "ABAB", here t = "AB". str1 consists of 3t & str2 consists of 2t. The GCD of 3 & 2 is 1. So the result is AB.

    // Example 2 # "LEET" & "CODE", here there is not t. So, return ""

    public string GcdOfStrings(string str1, string str2) {

        if (!(str1 + str2).Equals(str2 + str1)) {

            return "";

        }

​

        return str1.Substring(0, Solution.FindGCD(str1.Length, str2.Length));

    }

}

public class Program

{

    public static void Main()

    {

        Solution sol = new Solution();

        if(sol.GcdOfStrings("ABABAB", "ABAB") == "AB"){

            Console.WriteLine("Case 1 pass");

        } else {

            Console.WriteLine("Case 1 failed");

        }

        if(sol.GcdOfStrings("ABCABC", "ABC") == "ABC"){

            Console.WriteLine("Case 2 pass");

        } else {

            Console.WriteLine("Case 2 failed");

        }

        if(sol.GcdOfStrings("LEET", "CODE") == ""){

            Console.WriteLine("Case 3 pass");

        } else {

            Console.WriteLine("Case 3 failed");

        }

    }

}