**Prerequisites:** Dynamic Programming, DigitDP

Given two integers **N** and **K**. The task is to find the number of integers between **1** and **N** (inclusive) that contains exactly **K** non-zero digits when written in base ten.

**Examples:**

Input:N = 100, K = 1Output:19Explanation:

The digits with exactly 1 non zero digits between 1 and 100 are:

1, 2, 3, 4, 5, 6, 7, 8, 9,

10, 20, 30, 40, 50, 60, 70, 80, 90, 100

Input:N = 25, K = 2Output:14Explanation:

The digits with exactly 2 non zero digits between 1 and 25 are:

11, 12, 13, 14, 15, 16, 17,

18, 19, 21, 22, 23, 24, 25

**Approach:** It is enough to consider the integers of **N** digits, by filling the higher digits with **0** if necessary. This problem can be solved by applying the method called digit DP.

**dp[i][0][j]**= The higher**i**digits have already been decided, and there are**j**non-zero digits, and it has already been determined that it is less than**N**.**dp[i][1][j]**= The higher**i**digits have already been decided, and there are**j**non-zero digits, and it has not yet been determined that it is less than**N**.

After computing the above dp, the desired answer is **dp[L][0][K] + dp[L][1][K]**, where **L** is the number of digits of **N**.

Below is the implementation of the above approach:

## C++

`// C++ implementation of the above approach` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to find number less than N` `// having k non-zero digits` `int` `k_nonzero_numbers(string s, ` `int` `n, ` `int` `k)` `{` ` ` `// Store the memorised values` ` ` `int` `dp[n + 1][2][k + 1];` ` ` `// Initialise` ` ` `for` `(` `int` `i = 0; i <= n; i++)` ` ` `for` `(` `int` `j = 0; j < 2; j++)` ` ` `for` `(` `int` `x = 0; x <= k; x++)` ` ` `dp[i][j][x] = 0;` ` ` `// Base` ` ` `dp[0][0][0] = 1;` ` ` `// Calculate all states` ` ` `// For every state, from numbers 1 to N,` ` ` `// the count of numbers which contain exactly j` ` ` `// non zero digits is being computed and updated` ` ` `// in the dp array.` ` ` `for` `(` `int` `i = 0; i < n; ++i) {` ` ` `int` `sm = 0;` ` ` `while` `(sm < 2) {` ` ` `for` `(` `int` `j = 0; j < k + 1; ++j) {` ` ` `int` `x = 0;` ` ` `while` `(x <= (sm ? 9 : s[i] - ` `'0'` `)) {` ` ` `dp[i + 1][sm || x < (s[i] - ` `'0'` `)][j + (x > 0)]` ` ` `+= dp[i][sm][j];` ` ` `++x;` ` ` `}` ` ` `}` ` ` `++sm;` ` ` `}` ` ` `}` ` ` `// Return the required answer` ` ` `return` `dp[n][0][k] + dp[n][1][k];` `}` `// Driver code` `int` `main()` `{` ` ` `string s = ` `"25"` `;` ` ` `int` `k = 2;` ` ` `int` `n = s.size();` ` ` `// Function call` ` ` `cout << k_nonzero_numbers(s, n, k);` ` ` `return` `0;` `}` |

## Java

`// Java implementation of the above approach` `class` `Geeks{` `// Function to find number less than N` `// having k non-zero digits` `static` `int` `k_nonzero_numbers(String s, ` `int` `n, ` `int` `k)` `{` ` ` ` ` `// Store the memorised values` ` ` `int` `dp[][][] = ` `new` `int` `[n + ` `1` `][` `2` `][k + ` `1` `];` ` ` `// Initialise` ` ` `for` `(` `int` `i = ` `0` `; i <= n; i++)` ` ` `for` `(` `int` `j = ` `0` `; j < ` `2` `; j++)` ` ` `for` `(` `int` `x = ` `0` `; x <= k; x++)` ` ` `dp[i][j][x] = ` `0` `;` ` ` `// Base` ` ` `dp[` `0` `][` `0` `][` `0` `] = ` `1` `;` ` ` `// Calculate all states` ` ` `// For every state, from numbers 1 to N,` ` ` `// the count of numbers which contain exactly j` ` ` `// non zero digits is being computed and updated` ` ` `// in the dp array.` ` ` `for` `(` `int` `i = ` `0` `; i < n; ++i)` ` ` `{` ` ` `int` `sm = ` `0` `;` ` ` `while` `(sm < ` `2` `)` ` ` `{` ` ` `for` `(` `int` `j = ` `0` `; j < k + ` `1` `; ++j)` ` ` `{` ` ` `int` `x = ` `0` `;` ` ` `while` `(x <= (sm !=` ` ` `0` `? ` `9` `:s.charAt(i) - ` `'0'` `))` ` ` `{` ` ` `if` `(j + (x > ` `0` `? ` `1` `: ` `0` `) < k + ` `1` `)` ` ` `{` ` ` `dp[i + ` `1` `][(sm != ` `0` `|| x <` ` ` `(s.charAt(i) - ` `'0'` `)) ?` ` ` `1` `: ` `0` `][j + (x > ` `0` `?` ` ` `1` `: ` `0` `)] += dp[i][sm][j];` ` ` `}` ` ` `++x;` ` ` `}` ` ` `}` ` ` `++sm;` ` ` `}` ` ` `}` ` ` `// Return the required answer` ` ` `return` `dp[n][` `0` `][k] + dp[n][` `1` `][k];` `}` `// Driver code` `public` `static` `void` `main(String[] args)` `{` ` ` `String s = ` `"25"` `;` ` ` `int` `k = ` `2` `;` ` ` `int` `n = s.length();` ` ` `// Function call` ` ` `System.out.println(k_nonzero_numbers(s, n, k));` `}` `}` `// This code is contributed by Rajnis09` |

## Python3

`# Python3 implementation of the above approach` `# Function to find number less than N` `# having k non-zero digits` `def` `k_nonzero_numbers(s, n, k):` ` ` ` ` `# Store the memorised values` ` ` `dp ` `=` `[[[ ` `0` `for` `i ` `in` `range` `(k ` `+` `2` `)]` ` ` `for` `i ` `in` `range` `(` `2` `)]` ` ` `for` `i ` `in` `range` `(n ` `+` `2` `)]` ` ` `# Initialise` ` ` `for` `i ` `in` `range` `(n ` `+` `1` `):` ` ` `for` `j ` `in` `range` `(` `2` `):` ` ` `for` `x ` `in` `range` `(k ` `+` `1` `):` ` ` `dp[i][j][x] ` `=` `0` ` ` `# Base` ` ` `dp[` `0` `][` `0` `][` `0` `] ` `=` `1` ` ` `# Calculate all states` ` ` `# For every state, from numbers 1 to N,` ` ` `# the count of numbers which contain` ` ` `# exactly j non zero digits is being` ` ` `# computed and updated in the dp array.` ` ` `for` `i ` `in` `range` `(n):` ` ` `sm ` `=` `0` ` ` ` ` `while` `(sm < ` `2` `):` ` ` `for` `j ` `in` `range` `(k ` `+` `1` `):` ` ` `x ` `=` `0` ` ` `y ` `=` `0` ` ` `if` `sm:` ` ` `y ` `=` `9` ` ` `else` `:` ` ` `y ` `=` `ord` `(s[i]) ` `-` `ord` `(` `'0'` `)` ` ` `while` `(x <` `=` `y):` ` ` `dp[i ` `+` `1` `][(sm ` `or` `x < (` ` ` `ord` `(s[i]) ` `-` `ord` `(` `'0'` `)))][j ` `+` ` ` `(x > ` `0` `)] ` `+` `=` `dp[i][sm][j]` ` ` `x ` `+` `=` `1` ` ` ` ` `sm ` `+` `=` `1` ` ` `# Return the required answer` ` ` `return` `dp[n][` `0` `][k] ` `+` `dp[n][` `1` `][k]` `# Driver code` `if` `__name__ ` `=` `=` `'__main__'` `:` ` ` ` ` `s ` `=` `"25"` ` ` `k ` `=` `2` ` ` `n ` `=` `len` `(s)` ` ` `# Function call` ` ` `print` `(k_nonzero_numbers(s, n, k))` `# This code is contributed by mohit kumar 29` |

## C#

`// C# implementation of the above approach` `using` `System;` `using` `System.Collections;` `class` `GFG{` `// Function to find number less than N` `// having k non-zero digits` `static` `int` `k_nonzero_numbers(` `string` `s, ` `int` `n,` ` ` `int` `k)` `{` ` ` ` ` `// Store the memorised values` ` ` `int` `[,,]dp = ` `new` `int` `[n + 1, 2, k + 1];` ` ` `// Initialise` ` ` `for` `(` `int` `i = 0; i <= n; i++)` ` ` `for` `(` `int` `j = 0; j < 2; j++)` ` ` `for` `(` `int` `x = 0; x <= k; x++)` ` ` `dp[i, j, x] = 0;` ` ` `// Base` ` ` `dp[0, 0, 0] = 1;` ` ` `// Calculate all states` ` ` `// For every state, from numbers 1 to N,` ` ` `// the count of numbers which contain exactly j` ` ` `// non zero digits is being computed and updated` ` ` `// in the dp array.` ` ` `for` `(` `int` `i = 0; i < n; ++i)` ` ` `{` ` ` `int` `sm = 0;` ` ` `while` `(sm < 2)` ` ` `{` ` ` `for` `(` `int` `j = 0; j < k + 1; ++j)` ` ` `{` ` ` `int` `x = 0;` ` ` `while` `(x <= (sm !=` ` ` `0 ? 9 : s[i]- ` `'0'` `))` ` ` `{` ` ` `if` `(j + (x > 0 ? 1 : 0) < k + 1)` ` ` `{` ` ` `dp[i + 1, ((sm != 0 || x <` ` ` `(s[i] - ` `'0'` `)) ? 1 : 0),` ` ` `j + (x > 0 ? 1 : 0)] +=` ` ` `dp[i, sm, j];` ` ` `}` ` ` `++x;` ` ` `}` ` ` `}` ` ` `++sm;` ` ` `}` ` ` `}` ` ` `// Return the required answer` ` ` `return` `dp[n, 0, k] + dp[n, 1, k];` `}` `// Driver code` `public` `static` `void` `Main(` `string` `[] args)` `{` ` ` `string` `s = ` `"25"` `;` ` ` `int` `k = 2;` ` ` `int` `n = s.Length;` ` ` `// Function call` ` ` `Console.Write(k_nonzero_numbers(s, n, k));` `}` `}` `// This code is contributed by rutvik_56` |

**Output:**

14

**Time Complexity:** O(LK) where L is the number of digits in N. **Note:** The two for loops used to calculate the state which from [0, 1] and [0, 9] respectively are considered as a constant multiplication.

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