How many zeros are there behind the factorial result of n? such as the factorial of 2016 (NetEase's written selection questions)

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Author: User

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When the order multiplier is small, it can be calculated directly.

For example: Ask for 10. The number of the following 0.   The result is obviously 3628800. The number of 0 is 2.

20. The result is a number of 4 2432902008176640000,0. And this time has reached the 19-digit number.

A larger number, if you go straight to the request. It's obviously too much trouble.

Second, the law behind

This paper introduces a method to find the number of 0 after a certain number factorial result.

First, zero occurs when multiples of 5 are multiplied by even numbers.

But the number that appears 0 is not necessarily, 2*5=10 appears a 0. 4*25=100 will appear 2 0. 8*125=1000 appears 3 0.

It is easy to find the rules. 4*25 can be decomposed into 2*5*2*5,8*125 which can be decomposed into 2*5*2*5*2*5 ...

As for the 10,15,20 these numbers. Decomposition, only a 5 will appear, will only appear 1 0.

As for 50,75 these multiples of 25, decomposition, will only appear 2 5, and even multiplication will only appear 2 0.

Here is the count of 0.

As mentioned above, even times 5 will appear a 0, multiplied by 5 squared will appear 2 0, multiplied by 5 of the Cubic will appear 3 0.

So how do you calculate how many multiples of 5 are present in factorial, multiples of 25, and multiples of 125? Also, there is a problem to note that 25,125 is also a multiple of 5, 125 is also a multiple of 25.

For example, ask for 100. The number of the following 0.

The method mentioned above is that the number of multiples of 5 is 16 (minus the multiples of 25), the number of multiples of 25 is 4, and the result should be 16*1+4*2=24 of 0.

We can also change a thought, appear a 5 is a 0, then appear a 25 is 2 0, if the multiples of 5 does not exclude the multiples of 25, each appears a multiple of 25, only add a 0 can.

In other words, the number of 0 is equal to the number of multiples of 5 plus the number of multiples of 25, that is, 20+4.

If it is to ask 2016. The number of the back 0.

Also follow the above method.

The number of multiples of 5 is: 2016/5 = 403

The number of multiples of 25 is: 403/5 = 80

The number of multiples of 125 is: 80/5 = 16

The number of multiples of 625 is: 16/5 = 3.

So we can draw 2016. The number of the following 0 is: 403+80+16+3 = 502.

How to use the code and how to implement it.

Very simple, with a simple loop can be solved, of course, recursive algorithm is better. Here we just paste the loop code.

Scanner Scanner = new Scanner (;
System.out.print ("Please enter the Order multiplier:");
int a = Scanner.nextint ();
int count = 0;
for (;;) {
A = A/5;
if (a = = 0) {
}else {
count+= A;
System.out.println ("The number of 0 after the factorial result is:" +count);

The results are as follows:

Please enter the Order multiplier: 2016
The number of 0 after the factorial result is: 502

Of course, the weight in the mind, the code has nothing to say. I hope you have learned this method. Although it is of little use in life, the method of transforming ideas is very useful.

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