My implementation of the Permutation algorithm in Swift

Permutation.swift

class Permutator {
    class func permutation(_ str: String) -> Set<String> {
        return permutation(str, prefix: "")
    }
    
    private class func permutation(_ str: String, prefix: String) -> Set<String> {
        if str.characters.count == 0 {
            return [prefix]
        }
        
        var set = Set<String>()
        for i in str.characters.indices {
            
            let left    = str.substring(to: i)
            let right   = str.substring(from: str.index(after: i))
            
            let rem = left + right
            
            set = set.union(permutation(rem, prefix: prefix + String(str[i])))
        }
        
        return set
    }
}