e050.swift

// 준비작업 - 소수체를 생성하고, 누적합 리스트를 만든다.
let limit = 100_0000
let seive: [Bool] = {
  var s = [false, false] + Array(repeating: true, count:limit)
  for i in 2...limit where s[i] {
    for j in stride(from:i*2, through:limit, by:i) { s[j] = false }
  }
  return s
}()

let isPrime: (Int) -> Bool = { seive[$0] }
let primes: [Int] = (1...limit).filter(isPrime)

var sums: [Int] = {
  var xs = Array(repeating: 0, count: 1 + primes.count)
  for i in 1..<xs.count {
    xs[i] = xs[i-1] + primes[i-1]
  }
  return xs.filter{ $0 < limit * 2 }
}()

// 최대길이를 줄여나가고, 오프셋값을 늘려나가며 조건에 맞는 부분합을 찾는다.
main: do {
  let maxLength = sums.count - 1
  for l in stride(from: maxLength, to:0, by: -1) {
    for offset in 0..<(maxLength - l) {
      let gap = sums[l+offset+1] - sums[offset]
      if gap <= limit {
        if isPrime(gap){
          print(gap)
          break main
        }
      } else {
        break
      }
    }
  }
}