# Maximum subarray sum in an array created after repeated concatenation

• Difficulty Level : Medium
• Last Updated : 12 Apr, 2021

Given an array and a number k, find the largest sum of contiguous array in the modified array which is formed by repeating the given array k times.
Examples :

```Input  : arr[] = {-1, 10, 20}, k = 2
Output : 59
After concatenating array twice, we
get {-1, 10, 20, -1, 10, 20} which has
maximum subarray sum as 59.

Input  : arr[] = {-1, -2, -3}, k = 3
Output : -1```

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A simple solution is to create an array of size n*k, then run Kadane’s algorithm. Time complexity would be O(nk) and auxiliary space would be O(n*k)
A better solution is to run a loop on same array and use modular arithmetic to move back beginning after end of array.

## C++

 `// C++ program to print largest contiguous``// array sum when array is created after``// concatenating a small array k times.``#include``using` `namespace` `std;` `// Returns sum of maximum sum subarray created``// after concatenating a[0..n-1] k times.``int` `maxSubArraySumRepeated(``int` `a[], ``int` `n, ``int` `k)``{``    ``int` `max_so_far = INT_MIN, max_ending_here = 0;` `    ``for` `(``int` `i = 0; i < n*k; i++)``    ``{``        ``// This is where it differs from Kadane's``        ``// algorithm. We use modular arithmetic to``        ``// find next element.``        ``max_ending_here = max_ending_here + a[i%n];` `        ``if` `(max_so_far < max_ending_here)``            ``max_so_far = max_ending_here;` `        ``if` `(max_ending_here < 0)``            ``max_ending_here = 0;``    ``}``    ``return` `max_so_far;``}` `/*Driver program to test maxSubArraySum*/``int` `main()``{``    ``int` `a[] = {10, 20, -30, -1};``    ``int` `n = ``sizeof``(a)/``sizeof``(a);``    ``int` `k = 3;``    ``cout << ``"Maximum contiguous sum is "``         ``<< maxSubArraySumRepeated(a, n, k);``    ``return` `0;``}`

## Java

 `// Java program to print largest contiguous``// array sum when array is created after``// concatenating a small array k times.``import` `java.io.*;` `class` `GFG {``    ` `// Returns sum of maximum sum``// subarray created after``// concatenating a[0..n-1] k times.``static` `int` `maxSubArraySumRepeated(``int` `a[],``                             ``int` `n, ``int` `k)``{``    ``int` `max_so_far = ``0``;``    ``int` `INT_MIN, max_ending_here=``0``;` `    ``for` `(``int` `i = ``0``; i < n*k; i++)``    ``{``        ``// This is where it differs from``        ``// Kadane's algorithm. We use modular``        ``//  arithmetic to find next element.``        ``max_ending_here = max_ending_here +``                                    ``a[i % n];` `        ``if` `(max_so_far < max_ending_here)``            ``max_so_far = max_ending_here;` `        ``if` `(max_ending_here < ``0``)``            ``max_ending_here = ``0``;``    ``}``    ``return` `max_so_far;``}` `// Driver program to test maxSubArraySum``public` `static` `void` `main (String[] args) {``    ` `    ``int` `a[] = {``10``, ``20``, -``30``, -``1``};``    ``int` `n = a.length;``    ``int` `k = ``3``;``    ` `    ``System.out.println(``"Maximum contiguous sum is "``                   ``+ maxSubArraySumRepeated(a, n, k));``}` `}``    ` `// This code is contributed by vt_m`

## Python3

 `# Python program to print``# largest contiguous``# array sum when array``# is created after``# concatenating a small``# array k times.` `# Returns sum of maximum``# sum subarray created``# after concatenating``# a[0..n-1] k times.``def` `maxSubArraySumRepeated(a, n, k):` `    ``max_so_far ``=` `-``2147483648``    ``max_ending_here ``=` `0`` ` `    ``for` `i ``in` `range``(n``*``k):``    ` `        ``# This is where it``        ``# differs from Kadane's``        ``# algorithm. We use``        ``#  modular arithmetic to``        ``# find next element.``        ``max_ending_here ``=` `max_ending_here ``+` `a[i``%``n]`` ` `        ``if` `(max_so_far < max_ending_here):``            ``max_so_far ``=` `max_ending_here`` ` `        ``if` `(max_ending_here < ``0``):``            ``max_ending_here ``=` `0``    ` `    ``return` `max_so_far`` ` `# Driver program``# to test maxSubArraySum` `a ``=` `[``10``, ``20``, ``-``30``, ``-``1``]``n ``=` `len``(a)``k ``=` `3` `print``(``"Maximum contiguous sum is "``,``    ``maxSubArraySumRepeated(a, n, k))` `# This code is contributed``# by Anant Agarwal.`

## C#

 `// C# program to print largest contiguous``// array sum when array is created after``// concatenating a small array k times.``using` `System;` `class` `GFG {``    ` `// Returns sum of maximum sum``// subarray created after``// concatenating a[0..n-1] k times.``static` `int` `maxSubArraySumRepeated(``int` `[]a,``                                  ``int` `n,``                                  ``int` `k)``{``    ``int` `max_so_far = 0;``    ``int` `max_ending_here=0;` `    ``for` `(``int` `i = 0; i < n * k; i++)``    ``{``        ``// This is where it differs from``        ``// Kadane's algorithm. We use modular``        ``// arithmetic to find next element.``        ``max_ending_here = max_ending_here +``                                  ``a[i % n];` `        ``if` `(max_so_far < max_ending_here)``            ``max_so_far = max_ending_here;` `        ``if` `(max_ending_here < 0)``            ``max_ending_here = 0;``    ``}``    ``return` `max_so_far;``}` `// Driver Code``public` `static` `void` `Main ()``{``    ` `    ``int` `[]a = {10, 20, -30, -1};``    ``int` `n = a.Length;``    ``int` `k = 3;``    ` `    ``Console.Write(``"Maximum contiguous sum is "``                  ``+ maxSubArraySumRepeated(a, n, k));``}``}``    ` `// This code is contributed by nitin mittal.`

## PHP

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## Javascript

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Output :

`Maximum contiguous sum is 30`

Can we use the repeating property of array to get a better solution ?

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