Given two strings, a method to decide if one is permutation of the other.

checkPermutation.swift

import Cocoa

struct Counter {
    var table =  Dictionary<Character,Int>()
    
    mutating func add(_ key: Character) {
        let value = table[key] ?? 0
        table[key] = value + 1
    }
    
    mutating func remove(_ key: Character) {
        let value = table[key] ?? 0
        table[key] = value - 1
    }
    
    func permutation() ->  Bool {
        return !table.values.contains(where: {$0 != 0})
    }
    
}

func checkPermutation(word1: String, word2: String) -> Bool {
    guard word1.count == word2.count else { return false }
    var counter = Counter()
    var iteratorW1 = word1.makeIterator()
    var iteratorW2 = word2.makeIterator()
    while let char1 = iteratorW1.next(), let char2 = iteratorW2.next()  {
        counter.add(char1)
        counter.remove(char2)
    }
    return counter.permutation()
}

print(checkPermutation(word1: "abc", word2: "cba"))