Minimum count of numbers required ending with 7 to sum as a given number

Given an integer N, the task is to find the minimum count of numbers ending with 7 such that the sum of these numbers is N.

Examples:

Input: N = 38
Output: 4
7 + 7 + 7 + 17

Input: N = 46
Output: -1
46 cannot be represented as the sum
of integers ending with 7.

Input: N = 215
Output: 5
7 + 7 + 7 + 7 + 187



Approach:

  • First observation here is that every number greater than or equal to 70 can always be written as the sum of numbers all ending with 7. For example, for 82 the last digit is 2, so at least 6 numbers ending with 7 are required i.e. (7 * 6 = 42). An array hasharr[] can be created where hasharr[i] represents the minimum number of numbers required having the last digit as 7 so the resultant sum has the last digit as i.
  • If the number is less than 70 then N has to be checked whether it is less than the sum of the minimum number of numbers ending with digit seven 7. If it is then it is not possible and print -1, otherwise if it is greater or equal than it is possible.

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
const int TEN = 10;
  
// Function to return the count of
// minimum numbers ending with 7
// required such that the sum
// of these numbers is n
int minCount(int n)
{
  
    // hasharr[i] will store the minimum
    // numbers ending with 7 so that it
    // sums to number ending with digit i
    int hasharr[TEN] = { 10, 3, 6, 9, 2, 5, 8, 1, 4, 7 };
  
    // Its always possible to write numbers > 69
    // to write as numbers ending with 7
    if (n > 69)
        return hasharr[n % TEN];
    else {
  
        // If the number is atleast equal to the
        // sum of minimum numbers ending with 7
        if (n >= hasharr[n % TEN] * 7)
            return (hasharr[n % TEN]);
        else
            return -1;
    }
}
  
// Driver code
int main()
{
    int n = 38;
  
    cout << minCount(n);
  
    return 0;
}

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Java

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// Java implementation of the above approach 
class GFG {
      
// Function to return the count of 
// minimum numbers ending with 7 
// required such that the sum 
// of these numbers is n 
static int minCount(int n) 
      
    // hasharr[i] will store the minimum 
    // numbers ending with 7 so that it 
    // sums to number ending with digit i 
    int[] hasharr = { 10, 3, 6, 9, 2,
                       5, 8, 1, 4, 7 }; 
  
    // Its always possible to write  
    // numbers > 69 to write as 
    // numbers ending with 7 
    if (n > 69
        return hasharr[n % 10]; 
    else
    
          
        // If the number is atleast equal 
        // to the sum of minimum numbers 
        // ending with 7 
        if (n >= hasharr[n % 10] * 7
            return (hasharr[n % 10]); 
        else
            return -1
    
  
// Driver code 
public static void main (String[] args) 
    int n = 38
      
    System.out.println(minCount(n)); 
  
// This code is contributed by spp____

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Python3

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# Python3 implementation of the above approach 
  
# Function to return the count of 
# minimum numbers ending with 7 
# required such that the sum 
# of these numbers is n 
def minCount(n):
      
    # hasharr[i] will store the minimum 
    # numbers ending with 7 so that it 
    # sums to number ending with digit i 
    hasharr = [ 10, 3, 6, 9, 2
                 5, 8, 1, 4, 7 ]
  
    # Its always possible to write  
    # numbers > 69 to write as 
    # numbers ending with 7 
    if (n > 69): 
        return hasharr[n % 10]
    else
          
        # If the number is atleast equal 
        # to the sum of minimum numbers 
        # ending with 7 
        if (n >= hasharr[n % 10] * 7):
            return hasharr[n % 10]
        else:
            return -1
  
# Driver code
n = 38
  
print(minCount(n))
  
# This code is contributed by spp____

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C#

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// C# implementation of the above approach 
using System;
  
class GFG{
      
// Function to return the count of 
// minimum numbers ending with 7 
// required such that the sum 
// of these numbers is n 
static int minCount(int n) 
      
    // hasharr[i] will store the minimum 
    // numbers ending with 7 so that it 
    // sums to number ending with digit i 
    int[] hasharr = { 10, 3, 6, 9, 2,
                       5, 8, 1, 4, 7 }; 
  
    // Its always possible to write 
    // numbers > 69 to write as 
    // numbers ending with 7 
    if (n > 69) 
        return hasharr[n % 10]; 
    else 
    
  
        // If the number is atleast equal  
        // to the sum of minimum numbers 
        // ending with 7 
        if (n >= hasharr[n % 10] * 7) 
            return (hasharr[n % 10]); 
        else
            return -1; 
    
  
// Driver code 
public static void Main (String[] args) 
    int n = 38; 
      
    Console.WriteLine(minCount(n)); 
  
// This code is contributed by spp____

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Output:

4

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