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