Sergio Alves

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.

PHP Code

class Solution {

    /**
     * @param String $str1
     * @param String $str2
     * @return String
     */
    function gcdOfStrings($str1, $str2) {
        if (($str1 . $str2) !== ($str2 . $str1)) {
            return "";
        }

        $len1 = strlen($str1);
        $len2 = strlen($str2);

        while ($len2 != 0) {
            $temp = $len2;
            $len2 = $len1 % $len2;
            $len1 = $temp;
        }

        return substr($str1, 0, $len1);
    }
}