ruby 費式數列(fibonacci), 使用動態規劃法

fibonacci_dynaminc.rb

# 複雜度 O(n)
input = gets

def fib_dy(n, f = [0, 1])
  if n >= f.length
    for i in f.length..n
     f[i] = f[i-1] + f[i-2]
    end
  end
  return f[n]
end

puts fib_dy(input.to_i)