Find M-th number whose repeated sum of digits of a number is N

Given two positive integers N and M. The task is to find M-th number whose sum of digits of a number until sum becomes single digit is N.

Examples:

Input: N = 1, M = 3
Output: 19 
The first two numbers being 1 and 9.

Input: N = 2, M = 5
Output:  38 
The first four numbers being 2, 11, 20 and 29.

A naive approach is to iterate for all numbers and keep a count of number whose sum returns N.



An efficient approach is to find the summation of digits till it becomes single digits in O(1) that has been discussed here. Hence the formula to find M-th number will be:

Mth number: (M-1)*9 + N

Below is the implementation of the above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ program to Find m-th number whose
// sum of digits of a number until
// sum becomes single digit is N
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the M-th
// number whosesum till one digit is N
int findNumber(int n, int m)
{
    int num = (m - 1) * 9 + n;
    return num;
}
  
// Driver Code
int main()
{
  
    int n = 2, m = 5;
    cout << findNumber(n, m);
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java program to Find m-th number whose
// sum of digits of a number until
// sum becomes single digit is N
class GFG
{
  
// Function to find the M-th
// number whosesum till one digit is N
static int findNumber(int n, int m)
{
    int num = (m - 1) * 9 + n;
    return num;
}
  
// Driver Code
public static void main(String args[])
{
    int n = 2, m = 5;
    System.out.print(findNumber(n, m));
}
}
  
// This code is contributed
// by Akanksha Rai

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 program to Find m-th number 
# whose sum of digits of a number
# until sum becomes single digit is N 
  
# Function to find the M-th 
# number whosesum till one digit is N
def findNumber(n, m) :
      
    num = (m - 1) * 9 + n; 
    return num; 
  
# Driver Code 
if __name__ == "__main__"
  
    n = 2 ;
    m = 5
    print(findNumber(n, m))
      
# This code is contributed by Ryuga

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program to Find m-th number whose
// sum of digits of a number until
// sum becomes single digit is N
using System;
  
class GFG
{
  
// Function to find the M-th
// number whosesum till one digit is N
static int findNumber(int n, int m)
{
    int num = (m - 1) * 9 + n;
    return num;
}
  
// Driver Code
public static void Main()
{
    int n = 2, m = 5;
    Console.Write(findNumber(n, m));
}
}
  
// This code is contributed
// by Akanksha Rai

chevron_right


PHP

filter_none

edit
close

play_arrow

link
brightness_4
code

<?php
// PHP program to Find m-th number whose
// sum of digits of a number until
// sum becomes single digit is N
  
// number whosesum till one digit is N
function findNumber($n, $m)
{
    $num = ($m - 1) * 9 + $n;
    return $num;
}
  
// Driver Code
$n = 2; $m = 5;
echo findNumber($n, $m);
  
// This code is contributed
// by Akanksha Rai
?>

chevron_right


Output:

38


My Personal Notes arrow_drop_up

Striver(underscore)79 at Codechef and codeforces D

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : Akanksha_Rai, AnkitRai01