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Counting numbers of n digits that are monotone
  • Difficulty Level : Medium
  • Last Updated : 22 May, 2019

Call decimal number a monotone if:

   D[\, i]\, \leqslant D[\, i+1]\, 0 \leqslant i \leqslant |D| .

Write a program which takes positive number n on input and returns number of decimal numbers of length n that are monotone. Numbers can’t start with 0.

Examples :

Input : 1
Output : 9
Numbers are 1, 2, 3, ... 9

Input : 2
Output : 45
Numbers are 11, 12, 13, .... 22, 23
...29, 33, 34, ... 39.
Count is 9 + 8 + 7 ... + 1 = 45

Explanation: Let’s start by example of monotone numbers:\{111\}, \{123\}, \{12223333444\}



All those numbers are monotone as each digit on higher place is \geq than the one before it.
What are the monotone numbers are of length 1 and digits 1 or 2? It is question to ask yourself at the very beginning. We can see that possible numbers are:
 \{1\}, \{2\}

That was easy, now lets expand the question to digits 1, 2 and 3:
 \{1\}, \{2\}, \{3\}

Now different question, what are the different monotone numbers consisting of only 1 and length 3 are there?
\{111\}

Lets try now draw this very simple observation in 2 dimensional array for number of length 3, where first column is the length of string and first row is possible digits:
  \begin{array}{c c c c c c c c c c} & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 &\\ 1 & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 &\\ 2 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 &\\ 3 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 &\\ \end{array}

Let’s try to fill 3rd row 3rd column(number of monotone numbers consisting from numbers 1 or 2 with length 2). This should be: \{11\}, \{12\}, \{22\}
If we will look closer we already have subsets of this set i.e:
\{11\}, \{12\} – Monotone numbers that has length 2 and consist of 1 or 2
\{22\} – Monotone numbers of length 2 and consisting of number 2

We just need to add previous values to get the longer one.
Final matrix should look like this:

  \begin{array}{c c c c c c c c c c} & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 &\\ 1 & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 &\\ 2 & 1 & 3 & 6 & 10 & 15 & 21 & 28 & 36 & 47 &\\ 3 & 1 & 4 & 10 & 20 & 35 & 56 & 84 & 120 & 167 &\\ \end{array}

C++




// CPP program to count numbers of n digits
// that are  monotone.
#include <cstring>
#include <iostream>
  
// Considering all possible digits as
// {1, 2, 3, ..9}
int static const DP_s = 9;
  
int getNumMonotone(int len)
{
  
    // DP[i][j] is going to store monotone
    // numbers of length i+1 considering
    // j+1 digits.
    int DP[len][DP_s];
    memset(DP, 0, sizeof(DP));
  
    // Unit length numbers
    for (int i = 0; i < DP_s; ++i)
        DP[0][i] = i + 1;
  
    // Single digit numbers
    for (int i = 0; i < len; ++i)
        DP[i][0] = 1;
  
    // Filling rest of the entries in bottom
    // up manner.
    for (int i = 1; i < len; ++i)
        for (int j = 1; j < DP_s; ++j)
            DP[i][j] = DP[i - 1][j] + DP[i][j - 1];
  
    return DP[len - 1][DP_s - 1];
}
  
// Driver code.
int main()
{
    std::cout << getNumMonotone(10);
    return 0;
}

Java




// Java program to count numbers 
// of n digits that are monotone.
  
class GFG 
{
    // Considering all possible 
    // digits as {1, 2, 3, ..9}
    static final int DP_s = 9;
      
    static int getNumMonotone(int len)
    {
      
        // DP[i][j] is going to store 
        // monotone numbers of length 
        // i+1 considering j+1 digits.
        int[][] DP = new int[len][DP_s];
      
        // Unit length numbers
        for (int i = 0; i < DP_s; ++i)
            DP[0][i] = i + 1;
      
        // Single digit numbers
        for (int i = 0; i < len; ++i)
            DP[i][0] = 1;
      
        // Filling rest of the entries 
        // in bottom up manner.
        for (int i = 1; i < len; ++i)
            for (int j = 1; j < DP_s; ++j)
                DP[i][j] = DP[i - 1][j] 
                           + DP[i][j - 1];
      
        return DP[len - 1][DP_s - 1];
    }
      
    // Driver code.
    public static void main (String[] args) 
    {
        System.out.println(getNumMonotone(10));
    }
}
  
// This code is contributed by Ansu Kumari.

Python3




# Python3 program to count numbers of n 
# digits that are monotone.
  
# Considering all possible digits as
# {1, 2, 3, ..9}
DP_s = 9
  
def getNumMonotone(ln):
  
    # DP[i][j] is going to store monotone
    # numbers of length i+1 considering
    # j+1 digits.
    DP = [[0]*DP_s for i in range(ln)]
  
    # Unit length numbers
    for i in range(DP_s):
        DP[0][i] = i + 1
  
    # Single digit numbers
    for i in range(ln):
        DP[i][0] = 1
  
    # Filling rest of the entries  
    # in bottom up manner.
    for i in range(1, ln):
  
        for j in range(1, DP_s):
            DP[i][j] = DP[i - 1][j] + DP[i][j - 1]
  
    return DP[ln - 1][DP_s - 1]
  
  
# Driver code
print(getNumMonotone(10))
  
  
# This code is contributed by Ansu Kumari

C#




// C# program to count numbers 
// of n digits that are monotone.
using System;
  
class GFG 
{
    // Considering all possible 
    // digits as {1, 2, 3, ..9}
    static int DP_s = 9;
      
    static int getNumMonotone(int len)
    {
      
        // DP[i][j] is going to store 
        // monotone numbers of length 
        // i+1 considering j+1 digits.
        int[,] DP = new int[len,DP_s];
      
        // Unit length numbers
        for (int i = 0; i < DP_s; ++i)
            DP[0,i] = i + 1;
      
        // Single digit numbers
        for (int i = 0; i < len; ++i)
            DP[i,0] = 1;
      
        // Filling rest of the entries 
        // in bottom up manner.
        for (int i = 1; i < len; ++i)
            for (int j = 1; j < DP_s; ++j)
                DP[i,j] = DP[i - 1,j] 
                        + DP[i,j - 1];
      
        return DP[len - 1,DP_s - 1];
    }
      
    // Driver code.
    public static void Main () 
    {
        Console.WriteLine(getNumMonotone(10));
    }
}
  
// This code is contributed by vt_m.

PHP




<?php
// PHP program to count numbers 
// of n digits that are monotone.
function getNumMonotone($len)
{
    // Considering all possible
    // digits as {1, 2, 3, ..9}
    $DP_s = 9;
  
  
    // DP[i][j] is going to store 
    // monotone numbers of length 
    // i+1 considering j+1 digits.
    $DP = array(array_fill(0, $len, 0),
                array_fill(0, $len, 0));
  
    // Unit length numbers
    for ($i = 0; $i < $DP_s; ++$i)
        $DP[0][$i] = $i + 1;
  
    // Single digit numbers
    for ($i = 0; $i < $len; ++$i)
        $DP[$i][0] = 1;
  
    // Filling rest of the entries 
    // in bottom up manner.
    for ($i = 1; $i < $len; ++$i)
        for ($j = 1; $j < $DP_s; ++$j)
            $DP[$i][$j] = $DP[$i - 1][$j] + 
                          $DP[$i][$j - 1];
  
    return $DP[$len - 1][$DP_s - 1];
}
  
// Driver code
echo getNumMonotone(10);
  
// This code is contributed by mits
?>


Output :
43758

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