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).

You are given two strings str1 and str2, return the largest string x such that x divides both str1 and str2. If there is no such string, return "".

Example 1:

Input: str1 = "ABAB", str2 = "AB"

Output: "AB"

Example 2:

Input: str1 = "NANANA", str2 = "NANA"

Output: "NA"

Constraints:

• 1 <= str1.length, str2.length <= 1000
• str1 and str2 consist of English uppercase letters.

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