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Count N-digit numbers having sum of digits equal to a Prime Number

  • Last Updated : 09 Aug, 2021

Given a positive integer N, the task is to count the number of N-digit numbers whose sum of digits is a prime number.

Examples:

Input: N = 1
Output: 4
Explanation: [2, 3, 5, 7] are single digit numbers whose sum of digits is equal to a prime number.

Input : N = 2
Output : 33

Naive Approach: The simplest approach to solve the given problem is to generate all possible N-digit numbers and count those numbers whose sum of digits is a prime number. After checking for all the numbers, print the value of the count as the resultant total count of numbers. 



Time Complexity: O(N *10N)

Efficient Approach: The above approach can also be optimized by using Recursive Dynamic Programming because the above problem has overlapping subproblems and an optimal substructure. Follow the steps below to solve the problem:

  • Initialize a global 2D array dp[N+1][N*10] with all values as -1 that stores the result of each recursive call.
  • Compute prime numbers upto (N * 10) numbers by using Sieve of Eratosthenes.
  • Define a recursive function, say countOfNumbers(index, sum, N) by performing the following steps.
    • If the value of the index is N+1,
      • If the sum is a prime number, return 1 as a valid N-digit number has been formed.
      • Else return 0.
    • If the result of the state dp[index][sum] is already computed, return this value dp[index][sum].
    • If the current index is 1, then any digit from [1- 9] can be placed, else, [0-9] can be placed.
    • After making a valid placement of digits, recursively call the countOfNumbers function for the next index, and sum up all recursive pending results into variable val.
    • Store the value of val into the current state of the dp[index][sum] table.
    • Return this state’s result val to it’s parent recursive call.
  • Print the value returned by the function countOfNumbers(1, 0, N) as the result.

Below is the implementation of the above approach:

C++




#include <bits/stdc++.h>
using namespace std;
 
// Stores the dp states
int dp[100][1000];
 
// Check if a number is
// a prime or not
bool prime[1005];
 
// Function to generate all prime numbers
// that are less than or equal to n
void SieveOfEratosthenes(int n)
{
    // Base cases.
    prime[0] = prime[1] = false;
    for (int p = 2; p * p <= n; p++) {
 
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p] == true) {
 
            // Update all multiples
            // of as non-prime
            for (int i = p * p; i <= n; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to find the count of N-digit numbers
// such that the sum of digits is a prime number
int countOfNumbers(int index, int sum, int N)
{
 
    // If end of array is reached
    if (index == N + 1) {
 
        // If the sum is equal to a prime number
        if (prime[sum] == true) {
            return 1;
        }
 
        // Otherwise
        return 0;
    }
 
    int& val = dp[index][sum];
 
    // If the dp-states are already computed
    if (val != -1) {
 
        return val;
    }
 
    val = 0;
 
    // If index = 1, any digit from [1 - 9] can be placed.
    // If N = 1, 0 also can be placed.
    if (index == 1) {
 
        for (int digit = (N == 1 ? 0 : 1); digit <= 9;
             ++digit) {
            val += countOfNumbers(index + 1, sum + digit,
                                  N);
        }
    }
 
    // Otherwise, any digit from [0 - 9] can be placed.
    else {
        for (int digit = 0; digit <= 9; ++digit) {
            val += countOfNumbers(index + 1, sum + digit,
                                  N);
        }
    }
 
    // Return the answer.
    return val;
}
 
// Driver Code
int main()
{
    // Initializing dp array with -1
    memset(dp, -1, sizeof dp);
 
    // Initializing prime array to true
    memset(prime, true, sizeof(prime));
 
    // Find all primes less than or equal to 1000,
    // which is sufficient for N upto 100
    SieveOfEratosthenes(1000);
 
    // Given Input
    int N = 6;
 
    // Function call
    cout << countOfNumbers(1, 0, N);
 
    return 0;
}

Java




import java.util.Arrays;
 
class GFG{
 
// Stores the dp states
public static int[][] dp = new int[100][1000];
 
// Check if a number is
// a prime or not
public static boolean[] prime = new boolean[1005];
 
// Function to generate all prime numbers
// that are less than or equal to n
public static void SieveOfEratosthenes(int n)
{
     
    // Base cases.
    prime[0] = prime[1] = false;
    for(int p = 2; p * p <= n; p++)
    {
         
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p] == true)
        {
             
            // Update all multiples
            // of as non-prime
            for(int i = p * p; i <= n; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to find the count of N-digit numbers
// such that the sum of digits is a prime number
public static int countOfNumbers(int index, int sum,
                                 int N)
{
     
    // If end of array is reached
    if (index == N + 1)
    {
         
        // If the sum is equal to a prime number
        if (prime[sum] == true)
        {
            return 1;
        }
 
        // Otherwise
        return 0;
    }
 
    int val = dp[index][sum];
 
    // If the dp-states are already computed
    if (val != -1)
    {
        return val;
    }
 
    val = 0;
 
    // If index = 1, any digit from [1 - 9] can be placed.
    // If N = 1, 0 also can be placed.
    if (index == 1)
    {
        for(int digit = (N == 1 ? 0 : 1);
                digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
 
    // Otherwise, any digit from [0 - 9] can be placed.
    else
    {
        for(int digit = 0; digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
 
    // Return the answer.
    return val;
}
 
// Driver Code
public static void main(String args[])
{
     
    // Initializing dp array with -1
    for(int[] row : dp)
        Arrays.fill(row, -1);
 
    // Initializing prime array to true
    Arrays.fill(prime, true);
 
    // Find all primes less than or equal to 1000,
    // which is sufficient for N upto 100
    SieveOfEratosthenes(1000);
 
    // Given Input
    int N = 6;
 
    // Function call
    System.out.println(countOfNumbers(1, 0, N));
}
}
 
// This code is contributed by gfgking

C#




using System;
 
public class GFG{
 
// Stores the dp states
public static int[,] dp = new int[100,1000];
 
// Check if a number is
// a prime or not
public static bool[] prime = new bool[1005];
 
// Function to generate all prime numbers
// that are less than or equal to n
public static void SieveOfEratosthenes(int n)
{
     
    // Base cases.
    prime[0] = prime[1] = false;
    for(int p = 2; p * p <= n; p++)
    {
         
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p] == true)
        {
             
            // Update all multiples
            // of as non-prime
            for(int i = p * p; i <= n; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to find the count of N-digit numbers
// such that the sum of digits is a prime number
public static int countOfNumbers(int index, int sum,
                                 int N)
{
     
    // If end of array is reached
    if (index == N + 1)
    {
         
        // If the sum is equal to a prime number
        if (prime[sum] == true)
        {
            return 1;
        }
 
        // Otherwise
        return 0;
    }
 
    int val = dp[index,sum];
 
    // If the dp-states are already computed
    if (val != -1)
    {
        return val;
    }
 
    val = 0;
 
    // If index = 1, any digit from [1 - 9] can be placed.
    // If N = 1, 0 also can be placed.
    if (index == 1)
    {
        for(int digit = (N == 1 ? 0 : 1);
                digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
 
    // Otherwise, any digit from [0 - 9] can be placed.
    else
    {
        for(int digit = 0; digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
 
    // Return the answer.
    return val;
}
 
// Driver Code
public static void Main(String []args)
{
     
    // Initializing dp array with -1
    for(int i = 0; i < 100; i++)
    {
        for (int j = 0; j < 1000; j++) {
            dp[i,j] = -1;
        }
    }
 
    // Initializing prime array to true
    for (int i = 0; i < prime.Length; i++)
        prime[i] = true;
 
    // Find all primes less than or equal to 1000,
    // which is sufficient for N upto 100
    SieveOfEratosthenes(1000);
 
    // Given Input
    int N = 6;
 
    // Function call
    Console.WriteLine(countOfNumbers(1, 0, N));
}
}
 
// This code is contributed by Princi Singh

Javascript




<script>
 
// Stores the dp states
let dp = new Array(100).fill(0).map(() =>
         new Array(1000).fill(-1));
 
// Check if a number is
// a prime or not
let prime = new Array(1005).fill(true);
 
// Function to generate all prime numbers
// that are less than or equal to n
function SieveOfEratosthenes(n)
{
     
    // Base cases.
    prime[0] = prime[1] = false;
     
    for(let p = 2; p * p <= n; p++)
    {
         
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p] == true)
        {
             
            // Update all multiples
            // of as non-prime
            for(let i = p * p; i <= n; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to find the count of N-digit numbers
// such that the sum of digits is a prime number
function countOfNumbers(index, sum, N)
{
     
    // If end of array is reached
    if (index == N + 1)
    {
         
        // If the sum is equal to a prime number
        if (prime[sum] == true)
        {
            return 1;
        }
         
        // Otherwise
        return 0;
    }
     
    let val = dp[index][sum];
     
    // If the dp-states are already computed
    if (val != -1)
    {
        return val;
    }
     
    val = 0;
     
    // If index = 1, any digit from [1 - 9] can
    // be placed. If N = 1, 0 also can be placed.
    if (index == 1)
    {
        for(let digit = N == 1 ? 0 : 1;
                digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
     
    // Otherwise, any digit from [0 - 9]
    // can be placed.
    else
    {
        for(let digit = 0; digit <= 9; ++digit)
        {
            val += countOfNumbers(index + 1,
                                    sum + digit, N);
        }
    }
     
    // Return the answer.
    return val;
}
 
// Driver Code
 
// Find all primes less than or equal to 1000,
// which is sufficient for N upto 100
SieveOfEratosthenes(1000);
 
// Given Input
let N = 6;
 
// Function call
document.write(countOfNumbers(1, 0, N));
 
// This code is contributed by gfgking
 
</script>
Output: 
222638

 

Time Complexity: O(N2 * 10)
Auxiliary Space: O(N2)

 

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