Explanation

Because str1 + str2 == str2 + str1 they must have a gcd. For instance, if the string "abab" is added to "ab" we are left with "ababab" == "ababab". In either case, we are left with the same result. Thus they must have some common sub-string.

If the condition is true (i.e., the strings are equal when concatenated in both orders), the method returns a substring of str1 starting from index 0 and ending at the greatest common divisor (GCD) of the lengths of str1 and str2.

If the condition is false then there is not a substring. Meaning that the function will return an empty string "", denoting that there is no common divisor.

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Solution

class Solution {
public:
    string gcdOfStrings(string str1, string str2) {
        return (str1 + str2 == str2 + str1)?
        str1.substr(0, gcd(size(str1), size(str2))): "";
    }
};

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