Factorial, such as 3! That is to say, starting from 1, multiply it by 3 in turn, which is 3! = 1*2*3

For any positive integer n, there is n! = 1*2*3*……*N

For example, five people line up in a row. How many arrangements are there?

According to the permutation algorithm, there should be * * * 5*4*3*2* 1 permutation, and there are five at this time! It is more convenient to express.