6.swift

import Foundation

let lines = try 
    String(contentsOfFile: "input", encoding: .utf8)
    .components(separatedBy: .newlines)
    .split(separator: "")

var part1 = 0
for line in lines {
    var set = Set<Character>()
    let answers = line.joined(separator: "")
    let yes = answers.filter { set.insert($0).inserted }.count
    part1 += yes
}

print("Part 1: \(part1)")

var part2 = 0
for line in lines {
    let first = line.first ?? ""
    let yes = line.reduce(Set(first)) { (result, list) in 
        return result.intersection(list)
    }.count
    part2 += yes
}

print("Part 2: \(part2)")