Count N-Digit Integers with Strictly Increasing Digits in Python

Suppose we have a number n, we have to find the number of n-digit positive integers such that the digits are in strictly increasing order.

So, if the input is like n = 3, then the output will be 84, as numbers are 123, 124, 125, ..., 678,789

To solve this, we will follow these steps −

• if n < 9 is non-zero, then

  • return Combination (9Cn)

• otherwise,

  • return 0

Let us see the following implementation to get better understanding −

Example

 Live Demo

from math import factorial as f
class Solution:
   def solve(self, n):
      if n < 9:
         return f(9) / f(n) / f(9 - n)
      else:
         return 0

ob = Solution()
print(ob.solve(3))

Input

3

Output

84