Getting permutations of a Set of Strings recursively.

recursivePermutations.swift

import Foundation

func permutations(_ set: Set<String>) -> Set<String> {
    
    guard set.count != 0 else {
        
        return Set<String>([""])
    }
    
    return set.reduce(Set<String>()) { accumulator, element in
        
        accumulator.union(permutations(set.subtracting([element])).map { element + $0 } )
    }
}

let set = Set<String>(["a","b","c","d","e"])

print(permutations(set))

Incredibly, Swift's flatMap implementation for Sets SUCKS. That's why I have to use this fold instead.