백준 10870번 피보나치 수 5

https://www.acmicpc.net/problem/10870

let num = Int(readLine()!)!
var s1 = 0
var s2 = 1
var s3 = s1 + s2
if num == 0 {
  print(s1)
} else if num == 1 {
  print(s2)
} else {
  for _ in 0..<num-1 {
      s3 = s2 + s1
      s1 = s2
      s2 = s3
}
print(s3)
}