Find sum of the series 1+22+333+4444+…… upto n terms

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Given a number N. The task is to find the sum of the below series up to N-th term:

1 + 22 + 333 + 4444 + …up to n terms

Examples:

Input: N = 3
Output: 356

Input: N = 10 
Output: 12208504795

Approach:

$Let $$A_n = 1 + 22 + 333 + 4444 +......+ n( \overbrace{11...1} )$$ Then\\ $$A_n = \frac{10^{n+1}(9n-1)}{9^3}+\frac{10}{9^3}-\frac{n(n+1)}{18}$$ we define $S_n(x)=x+2x^2++3x^2+...+nx^n$\\\\ $9A_n$ $=9+2.99+3.999+....+n(99...99)$\\ $9A_n$ $=(10-1)+2(10^2-1)+3(10^3-1)+...+n(10^n-1)$\\ $9A_n$ $=10+2.10^2+3.10^3+...+n.10^n-(1+2+3+...+n)$\\ $9A_n$ $=S_n(10)-\frac{n(n+1)}{2}$\\\\ Now, \\ $\frac{S_n(x)}{x}= 1+ 2x+3x^2+...+nx^{n}$\\ $\frac{S_n(x)}{x}= \frac{d}{dx}(1+x+x^2+x^3+...+x^n) $\\ $\frac{S_n(x)}{x}= \frac{d}{dx}\left (\frac{x^{n+1}-1}{x-1}\right )$\\ $\frac{S_n(x)}{x}= \frac{nx^{n+1}-(n+1)x^n+1}{(x-1)^2}$\\\\ Finally, $$A_n=\frac{1}{9} \left (10 \left ( \frac{n10^{n+1}-(n+1)10^n+1}{9^2}\right )-\frac{n(n+1)}{2}\right )}$$ $$A_n=\frac{10n^{n+1}(9n-1)}{9^3}+\frac{10}{9^3}-\frac{n(n+1)}{18}$$ $$$

Below is the implementation of the above approach:

C++

// CPP program to find the sum 
// of given series
 
#include <iostream>
#include <math.h>
 
using namespace std;
 
// Function to calculate sum
int findSum(int n)
{
    // Return sum
    return (pow(10, n + 1) * (9 * n - 1) + 10) / 
                    pow(9, 3) - n * (n + 1) / 18;
}
 
// Driver code
int main()
{
    int n = 3;
 
    cout << findSum(n);
     
    return 0;
}

Java

// Java Program to find 
// Sum of first n terms
import java.util.*;
 
class solution
{
static int calculateSum(int n)
{
 
// Returning the final sum
return ((int)Math.pow(10, n + 1) * (9 * n - 1) + 10) / 
                (int)Math.pow(9, 3) - n * (n + 1) / 18;
}
 
// Driver code
public static void main(String ar[])
{
// no. of terms to find the sum
int n=3;
System.out.println("Sum= "+ calculateSum(n));
 
}
}
 
//This code is contributed by Surendra_Gangwar

Python 3

# Python program to find the sum of given series.
 
 
# Function to calculate sum
def solve_sum(n):
    # Return sum
    return (pow(10, n + 1)*(9 * n - 1)+10)/pow(9, 3)-n*(n + 1)/18
 
# driver code
n = 3
 
print(int(solve_sum(n)))

C#

// C# Program to find 
// Sum of first n terms
using System;
class solution
{
static int calculateSum(int n)
{
 
// Returning the final sum
return ((int)Math.Pow(10, n + 1) * (9 * n - 1) + 10) / 
                (int)Math.Pow(9, 3) - n * (n + 1) / 18;
}
 
// Driver code
public static void  Main()
{
// no. of terms to find the sum
int n=3;
Console.WriteLine("Sum= "+ calculateSum(n));
 
}
}
 
//This code is contributed by inder_verma.

PHP

<?php
// PHP program to find the sum 
// of given series
 
// Function to calculate sum
function findSum($n)
{
    // Return sum
    return (pow(10, $n + 1) * 
               (9 * $n - 1) + 10) / 
            pow(9, 3) - $n * ($n + 1) / 18;
}
 
// Driver code
 
$n = 3;
 
echo findSum($n);
 
// This code is contributed 
// by inder_verma.
?>

Javascript

<script>
// Javascript Program to find 
// Sum of first n terms
 
    function calculateSum( n)
    {
 
        // Returning the const sum
        return (parseInt(Math.pow(10, n + 1)) * (9 * n - 1) + 10) / 
                parseInt(Math.pow(9, 3)) - n * (n + 1) / 18;
     }
 
    // Driver code
     
    // no. of terms to find the sum
    let n = 3;
    document.write("Sum= " + calculateSum(n));
 
// This code is contributed by 29AjayKumar  
</script>

Output:

Time Complexity: O(logn), where n represents the given integer, as we have used pow function.
Auxiliary Space: O(1), no extra space is required, so it is a constant.

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