flatten nested integers by Swift

nested_integers.swift

import Foundation

indirect enum Exp {
  case num(Int)
  case list([Exp])
}

// [1,2,[3,[4,5],6],7,[[8,9]],10]
func flatten(_ e: Exp) -> [Int] {
  var res = [Int]()
  switch e {
  case .num(let n):
    res.append(n)
  case .list(let es):
    for ns in es.map({ flatten($0) }) {
      for n in ns { res.append(n) }
    }
  }
  return res
}

let e: Exp = .list([.num(1),.num(2),.list([.num(3),.list([.num(4),.num(5)]),.num(6)]),.num(7),.list([.list([.num(8),.num(9)])]),.num(10)])

print(flatten(e))