Epic - Seed Number

Find the seed of a number.

Eg : 1716 = 143*1*4*3 =1716 so 143 is the seed of 1716. find all possible seed for a given number.

辗转相除法，由性质可利用 sqrt(num) <= seed_number <= num/2 缩小范围。

def seed_number(num)
  seed = Math.sqrt(num).to_i
  while seed <= num / 2
    if num % seed == 0
      product = temp = seed
      while temp != 0
        product *= temp%10
        temp /= 10
      end
      return seed if product == num
    end
    seed += 1
  end
end