Find minimum possible digit sum after adding a number d

Last Updated : 20 Aug, 2022

Given a number n and a number d, we can add d to n as many times ( even 0 is possible ). The task is to find the minimum possible digit sum we can achieve by performing above operation.
Digit Sum is defined as the sum of digits of a number recursively until it is less than 10.
Examples:

Input: n = 2546, d = 124
Output: 1
2546 + 8*124 = 3538 
DigitSum(3538)=1

Input: n = 123, d = 3
Output: 3

Approach:

First observation here is to use %9 approach to find minimum possible digit sum of a number n. If modulo with 9 is 0 return 9 else return the remainder.
Second observation is, a+d*(9k+l) modulo 9 is equivalent to a+d*l modulo 9, therefore, the answer to the query will be available in either no addition or first 8 additions of d, after which the digit sum will repeat.

Below is the implementation of above approach:

C++

// C++ implementation of above approach
#include <bits/stdc++.h>
using namespace std;

// Function To find digitsum for a number
int digitsum(int n)
{
    // Logic for digitsum
    int r = n % 9;
    if (r == 0)
        return 9;
    else
        return r;
}

// Function to find minimum digit sum
int find(int n, int d)
{
    // Variable to store answer
    // Initialise by 10 as the answer
    // will always be less than 10
    int minimum = 10;


    // Values of digitsum will repeat after
    // i=8, due to modulo taken with 9
    for (int i = 0; i < 9; i++) {
        int current = (n + i * d);
        minimum = min(minimum, digitsum(current));
    }

    return minimum;
}

// Driver Code
int main()
{
    int n = 2546, d = 124;
    cout << "Minimum possible digitsum is :"
         << find(n, d);

    return 0;
}

Java

// Java implementation of above approach
import java.io.*;
public class gfg
{
    // Function To find digitsum for a number
public int digitsum(int n)
{
    // Logic for digitsum
    int r = n % 9;
    if (r == 0)
        return 9;
    else
        return r;
}

// Function to find minimum digit sum
public int find(int n, int d)
{
    // Variable to store answer
    // Initialise by 10 as the answer
    // will always be less than 10
    int minimum = 10;


    // Values of digitsum will repeat after
    // i=8, due to modulo taken with 9
    for (int i = 0; i < 9; i++) {
        int current = (n + i * d);
        minimum = Math.min(minimum, digitsum(current));
    }

    return minimum;
}
}

class geek
{
// Driver Code
public static void main(String[]args)
{
    gfg g = new gfg();
    int n = 2546, d = 124;
    System.out.println("Minimum possible digitsum is : "+ (g.find(n, d)));
}
}
//This code is contributed by shs..

Python3

# Python3 implementation of 
# above approach

# Function To find digitsum 
# for a number
def digitsum(n):

    # Logic for digitsum
    r = n % 9;
    if (r == 0):
        return 9;
    else:
        return r;

# Function to find minimum digit sum
def find(n, d):

    # Variable to store answer
    # Initialise by 10 as the answer
    # will always be less than 10
    minimum = 10;

    # Values of digitsum will 
    # repeat after i=8, due to 
    # modulo taken with 9
    for i in range(9):

        current = (n + i * d);
        minimum = min(minimum,
                      digitsum(current));

    return minimum;

# Driver Code
n = 2546; 
d = 124;
print("Minimum possible digitsum is :", 
                           find(n, d));

# This code is contributed by mits

// C# implementation of above approach
using System;
public class gfg
{
    // Function To find digitsum for a number
 public int digitsum(int n)
 {
    // Logic for digitsum
    int r = n % 9;
    if (r == 0)
        return 9;
    else
        return r;
 }

// Function to find minimum digit sum
 public int find(int n, int d)
 {
    // Variable to store answer
    // Initialise by 10 as the answer
    // will always be less than 10
    int minimum = 10;


    // Values of digitsum will repeat after
    // i=8, due to modulo taken with 9
    for (int i = 0; i < 9; i++) {
        int current = (n + i * d);
        minimum = Math.Min(minimum, digitsum(current));
    }

    return minimum;
 }
}

class geek
{
// Driver Code
 public static void Main()
 {
    gfg g = new gfg();
    int n = 2546, d = 124;
    Console.WriteLine("Minimum possible digitsum is : {0}", (g.find(n, d)));
    Console.Read();
 }
}
//This code is contributed by SoumikMondal

PHP

<?php
// PHP implementation of 
// above approach

// Function To find digitsum 
// for a number
function digitsum($n)
{
    // Logic for digitsum
    $r = $n % 9;
    if ($r == 0)
        return 9;
    else
        return $r;
}

// Function to find minimum digit sum
function find($n, $d)
{
    // Variable to store answer
    // Initialise by 10 as the answer
    // will always be less than 10
    $minimum = 10;

    // Values of digitsum will 
    // repeat after i=8, due to 
    // modulo taken with 9
    for ($i = 0; $i < 9; $i++) 
    {
        $current = ($n + $i * $d);
        $minimum = min($minimum, 
                   digitsum($current));
    }

    return $minimum;
}

// Driver Code
$n = 2546; $d = 124;
echo "Minimum possible digitsum is :", 
                         find($n, $d);

// This code is contributed 
// by Shashank
?>

JavaScript

<script>

// javascript implementation of above approach
 // Function To find digitsum for a number
 function digitsum( n)
 {
    // Logic for digitsum
    var r = n % 9;
    if (r == 0)
        return 9;
    else
        return r;
 }
  
// Function to find minimum digit sum
 function find( n,  d)
 {
    // Variable to store answer
    // Initialise by 10 as the answer
    // will always be less than 10
    var minimum = 10;
  
  
    // Values of digitsum will repeat after
    // i=8, due to modulo taken with 9
    for (var i = 0; i < 9; i++) {
        var current = (n + i * d);
        minimum = Math.min(minimum, digitsum(current));
    }
  
    return minimum;
 }

  


 var n = 2546, d = 124;
 document.write("Minimum possible digitsum is :" + find(n, d));
 
</script>

Output:

Minimum possible digitsum is :1

Time Complexity: O(9)

Auxiliary Space: O(1)

Finding sum of digits of a number until sum becomes single digit

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