Calculate all permutations of a string in Swift

Question

For the string \"ABC\" the code snippet below calculates 5 of the 6 total permutations. My strategy was to insert each character at each index possible index. B

Answer 1

A very straightforward approach as also suggested in Swift coding challenges.

 func permutation(string: String, current: String = "") {
        let length = string.characters.count
        let strArray = Array(string.characters)
        if (length == 0) {
            // there's nothing left to re-arrange; print the result
            print(current)
            print("******")
        } else {
            print(current)
            // loop through every character
            for i in 0 ..< length {
                // get the letters before me
                let left = String(strArray[0 ..< i])
                // get the letters after me
                let right = String(strArray[i+1 ..< length])
                // put those two together and carry on
                permutation(string: left + right, current: current +
                    String(strArray[i]))
            }

        }
    }

Answer 2

Here is my solution.

import Foundation

class Permutator {
    class func permutation(_ str: String) -> Set<String> {
         var set = Set<String>()

         permutation(str, prefix: "", set: &set)

         return set
    }

    private class func permutation(_ str: String, prefix: String, set: inout Set<String>) {
        if str.characters.count == 0 {
            set.insert(prefix)
        }

        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
            permutation(rem, prefix: prefix + String(str[i]), set: &set)
        }
    }
}


let startTime = Date()
let permutation = Permutator.permutation("abcdefgh")


print("\(permutation) \n")
print("COMBINAISON: \(permutation.count)")
print("TIME: \(String(format: "%.3f", Date().timeIntervalSince(startTime)))s")

You can copy/paste it in a file and execute it with the command line swift binary.

For a permutation of 7 all unique characters, this algorithm take around 0,06 second to execute.