#leetcode #array

1071. Greatest Common Divisor of Strings - Swift.swift

class Solution {
    func gcdOfStrings(_ str1: String, _ str2: String) -> String {
        if str2.count > str1.count {
            return findGCDOf(str1: str2, str2: str1)
        } else {
            return findGCDOf(str1: str2, str2: str1)
        }
    }
    
    func findGCDOf(str1: String, str2: String) -> String {
        // - WHY: Reversing the indexes so we go from big range to small. Otherwise we may find a smaller common devisor and return early.
        // - NOTE: `indices` doesn't return the `endIndex`. It only returns _valid_ indexes. So that's awesome. 
        for i in str2.indices.reversed() {
            let subString = String(str2[str1.startIndex...i])
            if str1.isDivisible(byRepeating: subString) && str2.isDivisible(byRepeating: subString) {
                return subString
            }
        }
        return ""
    }
}

extension String {
    func isDivisible(byRepeating str: String) -> Bool {
        var result = str
        while self.count > result.count {
            result += str
        }
        return result == self
    }
}