1071. Greatest Common Divisor of Strings

For two strings s and t, we say "t divides s" if and only if s = t + t + t + ... + t + t (i.e., t is concatenated with itself one or more times). Given two strings str1 and str2, return the largest string x such that x divides both str1 and str2. Example 1: Input: str1 = "ABCABC", str2 = "ABC" Output: "ABC" Example 2: Input: str1 = "ABABAB", str2 = "ABAB" Output: "AB" Example 3: Input: