GCDOf2Strings.py

# 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: str1 = "LEET", str2 = "CODE"
# Output: ""
 
# Constraints:

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

class Solution:
    def gcdOfStrings(self, str1: str, str2: str) -> str:
        if str1 + str2 != str2 + str1:
            return ""
        def gcd(a, b):
            while b:
                a, b = b, a % b
            return a
        return str1[:gcd(len(str1), len(str2))]