# Count of N-digit numbers with all distinct digits

Given an integer **N**, the task is to find the count of **N-digit** numbers with all distinct digits.

**Examples:**

Input:N = 1

Output:9

1, 2, 3, 4, 5, 6, 7, 8 and 9 are the 1-digit numbers

with all distinct digits.

Input:N = 3

Output:648

**Approach:** If **N > 10** i.e. there will be atleast one digit which will be repeating hence for such cases the answer will be **0** else for the values of **N = 1, 2, 3, …, 9**, a series will be formed as **9, 81, 648, 4536, 27216, 136080, 544320, …** whose **N ^{th}** term will be

**9 * 9! / (10 – N)!**.

Below is the implementation of the above approach:

## C++

`// C++ implementation of the approach ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Function to return the factorial of n ` `int` `factorial(` `int` `n) ` `{ ` ` ` `if` `(n == 0) ` ` ` `return` `1; ` ` ` `return` `n * factorial(n - 1); ` `} ` ` ` `// Function to return the count ` `// of n-digit numbers with ` `// all distinct digits ` `int` `countNum(` `int` `n) ` `{ ` ` ` `if` `(n > 10) ` ` ` `return` `0; ` ` ` `return` `(9 * factorial(9) ` ` ` `/ factorial(10 - n)); ` `} ` ` ` `// Driver code ` `int` `main() ` `{ ` ` ` `int` `n = 3; ` ` ` ` ` `cout << countNum(n); ` ` ` ` ` `return` `0; ` `} ` |

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## Java

`// Java implementation of the approach ` `class` `GFG ` `{ ` ` ` ` ` `// Function to return the factorial of n ` ` ` `static` `int` `factorial(` `int` `n) ` ` ` `{ ` ` ` `if` `(n == ` `0` `) ` ` ` `return` `1` `; ` ` ` `return` `n * factorial(n - ` `1` `); ` ` ` `} ` ` ` ` ` `// Function to return the count ` ` ` `// of n-digit numbers with ` ` ` `// all distinct digits ` ` ` `static` `int` `countNum(` `int` `n) ` ` ` `{ ` ` ` `if` `(n > ` `10` `) ` ` ` `return` `0` `; ` ` ` `return` `(` `9` `* factorial(` `9` `) / ` ` ` `factorial(` `10` `- n)); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main(String []args) ` ` ` `{ ` ` ` `int` `n = ` `3` `; ` ` ` `System.out.println(countNum(n)); ` ` ` `} ` `} ` ` ` `// This code is contributed by Srathore ` |

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## Python3

`# Python3 implementation of the approach ` ` ` `# Function to return the factorial of n ` `def` `factorial(n) : ` ` ` ` ` `if` `(n ` `=` `=` `0` `) : ` ` ` `return` `1` `; ` ` ` `return` `n ` `*` `factorial(n ` `-` `1` `); ` ` ` `# Function to return the count ` `# of n-digit numbers with ` `# all distinct digits ` `def` `countNum(n) : ` ` ` `if` `(n > ` `10` `) : ` ` ` `return` `0` `; ` ` ` ` ` `return` `(` `9` `*` `factorial(` `9` `) ` `/` `/` ` ` `factorial(` `10` `-` `n)); ` ` ` `# Driver code ` `if` `__name__ ` `=` `=` `"__main__"` `: ` ` ` ` ` `n ` `=` `3` `; ` ` ` ` ` `print` `(countNum(n)); ` ` ` `# This code is contributed by AnkitRai01 ` |

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## C#

`// C# implementation of the approach ` `using` `System; ` ` ` `class` `GFG ` `{ ` ` ` ` ` `// Function to return the factorial of n ` ` ` `static` `int` `factorial(` `int` `n) ` ` ` `{ ` ` ` `if` `(n == 0) ` ` ` `return` `1; ` ` ` `return` `n * factorial(n - 1); ` ` ` `} ` ` ` ` ` `// Function to return the count ` ` ` `// of n-digit numbers with ` ` ` `// all distinct digits ` ` ` `static` `int` `countNum(` `int` `n) ` ` ` `{ ` ` ` `if` `(n > 10) ` ` ` `return` `0; ` ` ` `return` `(9 * factorial(9) / ` ` ` `factorial(10 - n)); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `Main(String []args) ` ` ` `{ ` ` ` `int` `n = 3; ` ` ` `Console.WriteLine(countNum(n)); ` ` ` `} ` `} ` ` ` `// This code is contributed by Princi Singh ` |

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**Output:**

648

**Time Complexity:** O(n)

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