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Count ways to represent N as sum of palindromic integers which do not have digit 1

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  • Difficulty Level : Hard
  • Last Updated : 09 Sep, 2022
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Given a positive integer N, the task is to find the number of distinct ways to express N as a sum of positive palindromic integers which do not have the digit 1 in them.

Note: As the answer can be quite large, print it modulo 109+7.

Examples:

Input: N = 4
Output: 2
Explanation: Following are the 2 Multisets that satisfy the condition:
1. 2+2 = 4
2. 4 = 4

Input: N = 8
Output: 7
Explanation: Following are the distinct Multisets that satisfy the condition:
1. 2+2+2+2 = 8
2. 2+3+3 = 8
3. 2+2+4 = 8
4. 4+4 = 8
5. 3+5 = 8
6. 2+6 = 8
7. 8 = 8

Approach:

Here each palindromic integers that don’t have the digit 1 can come an infinite number of times. (Repetition allowed), this is what we call Unbounded Knapsack

  • We have 2 choices for a palindromic number without a digit 1, either i) to include, or ii) to exclude.  But here, the inclusion process is not for just once; we can include any palindromic number without a single one any number of times until N < Sum.
  • Basically, If we are at V[m], we can take as many instances of that integer ( unbounded inclusion ) i.e count(V, m, sum – V[m] )  then we move to V[m-1]. After moving to V[m-1], we can’t move back and can’t make choices for V[m] i.e count(V, m-1, sum).
  • To find the total number of ways, so we have to add these 2 possible choices, i.e count(V, m, sum – S[m] ) + count(V, m-1, sum ).

Follow the steps given below for a better approach:

  • Declare a vector V to store all the numbers less than N which are palindrome and do not contain the digit 1.
  • Use a recursive function and in each recursive call, pass the vector V and the index integer(m) and the value of N (sum).
  • In each iteration, perform two recursive calls,  
    • In one of them decrease m(index element) by 1 and 
    • On the other one decrease the value of sum by V[m].
      • If the sum became 0 then return 1. So it will be added to the final answer which we return.
    • If sum or m is less than 0, then return 0.

Below is the implementation of the above approach.
 

C++




// C++ code to implement the approach
 
#include <bits/stdc++.h>
using namespace std;
 
int mod = 1e9 + 7;
 
// Function for checking if
// the given integer is palindrome or not
bool isPalindrome(int i)
{
    string x = to_string(i);
    int n = x.length();
    int cnt_one = 0;
    for (int i = 0; i < n / 2; i++) {
        if (x[i] == '1') {
            cnt_one++;
        }
        if (x[i] == x[n - 1 - i]) {
            continue;
        }
        else {
            return false;
        }
    }
    if (cnt_one == 1)
        return false;
    return true;
}
 
// Helper function
int helper(vector<int>& V, int m, int sum)
{
    // Base cases
 
    // As there is 1 way which is the empty set
    if (sum == 0)
        return 1;
 
    // If sum is negative then there is no way
    if (sum < 0)
        return 0;
 
    // If no integer left and the sum is not 0
    // then we cannot make sum 0
    if (m < 0 && sum > 0)
        return 0;
 
    return (helper(V, m - 1, sum) % mod
            + helper(V, m, sum - V[m]) % mod)
           % mod;
}
 
// Function to find the count of ways
int countMultiSet(int N)
{
    vector<int> V;
 
    // Vector storing the palindromic number till n
    for (int i = 2; i <= N; i++) {
        if (isPalindrome(i)) {
            V.push_back(i);
        }
    }
 
    // Calling the helper function
    return helper(V, V.size() - 1, N);
}
 
// driver function
int main()
{
    int N = 8;
 
    // calling the function
    cout << countMultiSet(N);
    return 0;
}

Java




/*package whatever //do not write package name here */
import java.io.*;
import java.util.ArrayList;
 
class GFG {
 
  // Function for checking if
  // the given integer is palindrome or not
  static boolean isPalindrome(Integer i)
  {
    String x = i.toString();
    int n = x.length();
    int cnt_one = 0;
    for (int j = 0; j < n / 2; i++) {
      if (x.charAt(j) == '1') {
        cnt_one++;
      }
      if (x.charAt(j) == x.charAt(n - 1- j)) {
        continue;
      }
      else {
        return false;
      }
    }
    if (cnt_one == 1)
      return false;
    return true;
  }
 
  // Helper function
  static int helper(ArrayList<Integer> V, int m, int sum)
  {
    // Base cases
 
    // As there is 1 way which is the empty set
    if (sum == 0)
      return 1;
 
    // If sum is negative then there is no way
    if (sum < 0)
      return 0;
 
    // If no integer left and the sum is not 0
    // then we cannot make sum 0
    if (m < 0 && sum > 0)
      return 0;
 
    return (helper(V, m - 1, sum) % (1000000007)
            + helper(V, m, sum - V.get(m)) % (1000000007))
      % (1000000007);
  }
 
  // Function to find the count of ways
  static int countMultiSet(int N)
  {
    ArrayList<Integer> V = new ArrayList<Integer>();
 
    // List storing the palindromic number till n
    for (int i = 2; i <= N; i++) {
      if (isPalindrome(i)) {
        V.add(i);
      }
    }
 
    // Calling the helper function
    return helper(V, V.size() - 1, N);
  }
    public static void main (String[] args) {
       int N = 8;
 
      // calling the function
      System.out.println(countMultiSet(N));
    }
}
 
// This code is contributed by hrithikgarg03188.

Python3




# Python3 code to implement the approach
mod = 1e9 + 7
 
# Function for checking if
# the given integer is palindrome or not
def isPalindrome(i):
    x = str(i)
    n = len(x)
    cnt_one = 0
 
    for i in range(0, int(n/2)):
 
        if (x[i] is '1'):
            cnt_one = cnt_one+1
        if (x[i] is x[n - 1 - i]):
            continue
        else:
            return false
    if (cnt_one is 1):
        return False
    return True
# Helper function
def helper(V, m, sum):
    # Base cases
 
    # As there is 1 way which is the empty set
    if (sum is 0):
        return 1
 
    # If sum is negative then there is no way
    if (sum < 0):
        return 0
 
    # If no integer left and the sum is not 0
    # then we cannot make sum 0
    if (m < 0 and sum > 0):
        return 0
 
    return (helper(V, m - 1, sum) % mod + helper(V, m, sum - V[m]) % mod) % mod
 
# Function to find the count of ways
def countMultiSet(N):
    V = []
 
    # Vector storing the palindromic number till n
    for i in range(2, N+1):
        if (isPalindrome(i)):
            V.append(i)
 
    # Calling the helper function
    return helper(V, len(V) - 1, N)
 
# driver function
N = 8
 
# calling the function
ans = countMultiSet(N)
print(int(ans))
 
# This code is contributed by akashish__

C#




using System;
using System.Collections.Generic;
 
public class GFG {
 
  // Function for checking if
  // the given integer is palindrome or not
  public static bool isPalindrome(int i)
  {
    string x = i.ToString();
    int n = x.Length;
    int cnt_one = 0;
    for (int j = 0; j < n / 2; i++) {
      if (x[j] == '1') {
        cnt_one++;
      }
      if (x[j] == x[n - 1 - j]) {
        continue;
      }
      else {
        return false;
      }
    }
    if (cnt_one == 1)
      return false;
    return true;
  }
 
  // Helper function
  public static int helper(List<int> V, int m, int sum)
  {
    // Base cases
 
    // As there is 1 way which is the empty set
    if (sum == 0)
      return 1;
 
    // If sum is negative then there is no way
    if (sum < 0)
      return 0;
 
    // If no integer left and the sum is not 0
    // then we cannot make sum 0
    if (m < 0 && sum > 0)
      return 0;
 
    return (helper(V, m - 1, sum) % (1000000007)
            + helper(V, m, sum - V[m]) % (1000000007))
      % (1000000007);
  }
 
  // Function to find the count of ways
  public static int countMultiSet(int N)
  {
    List<int> V = new List<int>();
 
    // List storing the palindromic number till n
    for (int i = 2; i <= N; i++) {
      if (isPalindrome(i)) {
        V.Add(i);
      }
    }
 
    // Calling the helper function
    return helper(V, V.Count - 1, N);
  }
 
  static public void Main()
  {
 
    int N = 8;
 
    // calling the function
    Console.WriteLine(countMultiSet(N));
  }
}
 
// This code is contributed by akashish__

Javascript




<script>
        // JavaScript code for the above approach
 
  // Function for checking if
  // the given integer is palindrome or not
  function isPalindrome(i)
  {
    let x = i.toString();
    let n = x.length;
    let cnt_one = 0;
    for (let j = 0; j < Math.floor(n / 2); i++) {
      if (x.charAt(j) == '1') {
        cnt_one++;
      }
      if (x.charAt(j) == x.charAt(n - 1- j)) {
        continue;
      }
      else {
        return false;
      }
    }
    if (cnt_one == 1)
      return false;
    return true;
  }
 
  // Helper function
  function helper(V,  m,  sum)
  {
    // Base cases
 
    // As there is 1 way which is the empty set
    if (sum == 0)
      return 1;
 
    // If sum is negative then there is no way
    if (sum < 0)
      return 0;
 
    // If no integer left and the sum is not 0
    // then we cannot make sum 0
    if (m < 0 && sum > 0)
      return 0;
 
    return (helper(V, m - 1, sum) % (1000000007)
            + helper(V, m, sum - V[m]) % (1000000007))
      % (1000000007);
  }
 
  // Function to find the count of ways
  function countMultiSet( N)
  {
    let V = new Array();
 
    // List storing the palindromic number till n
    for (let i = 2; i <= N; i++) {
      if (isPalindrome(i)) {
        V.push(i);
      }
    }
 
    // Calling the helper function
    return helper(V, V.length - 1, N);
  }
 
// Driver Code
    let N = 8;
 
      // calling the function
      document.write(countMultiSet(N));
       
      // This code is contributed by sanjoy_62.
    </script>

Output

7

Time Complexity: O(2M) M is the size of the vector V 
Auxiliary Space: O(M) 


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