1x2x3......x999 997x999 998x999 999x1 000 000

If you don't really need an exact number for that, Stirling's give you a pretty good approximation :)

http://mathworld.wolfram.com/StirlingsApproximation.html

1,000,000! ~= e^(1,000,000 ln 1,000,000 - 1,000,000)

In this way, it is possible to count the number of digits in the number.

如果真的要計算,會出現很大的數字…
所以用1 000 000 ！來表示可以是答案