1071. Greatest Common Divisor of Strings

Problem

For two strings s and t, we say "t divides s" if and only if s = 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.

Solution (Java)

class Solution {
    public String gcdOfStrings(String str1, String str2) {
        if (str1 == null || str2 == null) {
            return "";
        }
        if (str1.equals(str2)) {
            return str1;
        }
        int m = str1.length();
        int n = str2.length();
        if (m > n && str1.substring(0, n).equals(str2)) {
            return gcdOfStrings(str1.substring(n), str2);
        }
        if (n > m && str2.substring(0, m).equals(str1)) {
            return gcdOfStrings(str2.substring(m), str1);
        }
        return "";
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).