0312-2020-LEETCODE-1071- greatest common factor of the string (GCD algorithm)

Source: https: //leetcode-cn.com/problems/greatest-common-divisor-of-strings/solution/1071-zi-fu-chuan-de-zui-da-gong-yin-zi-by-wonderfu/

    public String gcdOfStrings(String str1, String str2) {
        int len1 = str1.length();
        int len2 = str2.length();
        if (len1 % len2 == 0){
            if (isGcd(str1,str2)){
                return str2;
            } else {
                return "";
            }
        }
        if (len2 % len1 == 0){
            if (isGcd(str2,str1)){
                return str1;
            } else {
                return "";
            }
        }
        TreeMap<Integer, String> map1 = new TreeMap<>();
        int j = len1 - 1;
        while (true){
            if (len1 % j == 0 && isGcd(str1,str1.substring(0,j))){
                map1.put(str1.substring(0,j).length(),str1.substring(0,j));
            }
            j--;
            if (j == 0){
                break;
            }
        }
        if (map1.size() == 0){
            return "";
        }
        TreeMap<Integer, String> map2 = new TreeMap<>();
        int i = len2 - 1;
        while (true){
            if (len2 % i == 0 && isGcd(str2,str2.substring(0,i))){
                map2.put(str2.substring(0,i).length(),str2.substring(0,i));
            }
            i--;
            if (i == 0){
                break;
            }
        }
        if (map2.size() == 0){
            return "";
        }
        ArrayList<String> list = new ArrayList<>();
        for (Map.Entry<Integer,String> entry : map2.entrySet()) {
            if (map1.containsKey(entry.getKey())){
                if (map1.get(entry.getKey()).equals(entry.getValue())){
                    list.add(entry.getValue());
                }
            }
        }
        return list.get(list.size()-1);
    }
    public boolean isGcd(String str1,String str2){
        while (str1.startsWith(str2)){
            if (str1.equals(str2)){
                return true;
            }
            str1 = str1.substring(str2.length());
        }
        return false;
    }
    //最大公因子算法gcd不会，只会笨办法.
    //官方题解
    public String gcdOfStrings1(String str1,String str2){
        if (!(str1 + str2).equals(str2 + str1)){
            return "";
        }
        return str1.substring(0,gcd(str1.length(),str2.length()));
    }

    private int gcd(int a, int b) {
        return b == 0 ? a : gcd(b,a % b);
    }