Maximum subarray sum in array formed by repeating the given array k times

Given an integer k and an integer array arr[] of n elements, the task is to find the largest sub-array sum in the modified array (formed by repeating the given array k times). For example, if arr[] = {1, 2} and k = 3 then modified array will be {1, 2, 1, 2, 1, 2}.

Examples:

Input: arr[] = {1, 2}, k = 3
Output: 9
Modified array will be {1, 2, 1, 2, 1, 2}
And the maximum sub-array sum will be 1 + 2 + 1 + 2 + 1 + 2 = 9

Input: arr[] = {1, -2, 1}, k = 5
Output: 2



A simple solution is to create an array of size n * k, then run Kadane’s algorithm to find the maximum sub-array sum. Time complexity would be O(n * k) with auxiliary space O(n * k).

A better solution is to calculate the sum of the array arr[] and store it in sum.

  • If sum < 0 then calculate the maximum sub-array sum of an array formed by concatenating the the array two times irrespective of the K. For example, take arr[] = {1, -4, 1} and k = 5. The sum of the array is less than 0. So, the maximum sub-array sum of the array can be found after concatenating the array two times only irrespective of the value of K i.e. b[] = {1, -4, 1, 1, -4, 1} and the maximum sub-array sum = 1 + 1 = 2
  • .

  • If sum > 0 then maximum sub-array will include the maximum sum as calculated in the previous step (where the array was concatenated twice) + the rest (k – 2) repetitions of the array can also be included as their sum is greater than 0 that will contribute to the maximum sum.

Below is the implementation of the above approach:

Java

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// Java implementation of the approach
public class GFG {
  
    // Function to concatenate array
    static void arrayConcatenate(int arr[], int b[], 
                                             int k)
    {
        // Array b will be formed by concatenation
        // array a exactly k times
        int j = 0;
        while (k > 0) {
  
            for (int i = 0; i < arr.length; i++) {
                b[j++] = arr[i];
            }
            k--;
        }
    }
  
    // Function to return the maximum 
    // subarray sum of arr[]
    static int maxSubArrSum(int a[])
    {
        int size = a.length;
        int max_so_far = Integer.MIN_VALUE,
            max_ending_here = 0;
  
        for (int i = 0; i < size; i++) {
            max_ending_here = max_ending_here + a[i];
            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;
    }
  
    // Function to return the maximum sub-array 
    // sum of the modified array
    static long maxSubKSum(int arr[], int k)
    {
        int arrSum = 0;
        long maxSubArrSum = 0;
  
        int b[] = new int[(2 * arr.length)];
  
        // Concatenating the array 2 times
        arrayConcatenate(arr, b, 2);
  
        // Finding the sum of the array
        for (int i = 0; i < arr.length; i++)
            arrSum += arr[i];
  
        // If sum is less than zero
        if (arrSum < 0)
            maxSubArrSum = maxSubArrSum(b);
  
        // If sum is greater than zero
        else
            maxSubArrSum = maxSubArrSum(b) +
                          (k - 2) * arrSum;
  
        return maxSubArrSum;
    }
  
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 1, -2, 1 };
        int k = 5;
        System.out.println(maxSubKSum(arr, k));
    }
}

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Below is the implementation of the above approach:

Python

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# Python approach to this problem
  
# A python module where element 
# are added to list k times
def MaxsumArrKtimes(c, ktimes):
      
    # Store element in list d k times
    d = c * ktimes
      
    # two variable which can keep 
    # track of maximum sum seen
    # so far and maximum sum ended.
    maxsofar = -99999
    maxending = 0
      
    for i in d:
        maxending = maxending + i
        if maxsofar < maxending:
            maxsofar = maxending
        if maxending < 0:
            maxending = 0
    return maxsofar
      
# Get the Maximum sum of element
print(MaxsumArrKtimes([1, -2, 1], 5))
      
# This code is contributed by AnupGaurav.
        

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C#

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// C# implementation of the approach 
using System;
  
class GFG 
  
// Function to concatenate array 
static void arrayConcatenate(int []arr, 
                             int []b, int k) 
    // Array b will be formed by concatenation 
    // array a exactly k times 
    int j = 0; 
    while (k > 0)
    
        for (int i = 0; i < arr.Length; i++) 
        
            b[j++] = arr[i]; 
        
        k--; 
    
  
// Function to return the maximum 
// subarray sum of arr[] 
static int maxSubArrSum(int []a) 
    int size = a.Length; 
    int max_so_far = int.MinValue, 
        max_ending_here = 0; 
  
    for (int i = 0; i < size; i++) 
    
        max_ending_here = max_ending_here + a[i]; 
        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; 
  
// Function to return the maximum sub-array 
// sum of the modified array 
static long maxSubKSum(int []arr, int k) 
    int arrSum = 0; 
    long maxSubArrsum = 0; 
  
    int []b = new int[(2 * arr.Length)]; 
  
    // Concatenating the array 2 times 
    arrayConcatenate(arr, b, 2); 
  
    // Finding the sum of the array 
    for (int i = 0; i < arr.Length; i++) 
        arrSum += arr[i]; 
  
    // If sum is less than zero 
    if (arrSum < 0) 
        maxSubArrsum = maxSubArrSum(b); 
  
    // If sum is greater than zero 
    else
        maxSubArrsum = maxSubArrSum(b) + 
                       (k - 2) * arrSum; 
  
    return maxSubArrsum; 
  
// Driver code 
public static void Main() 
    int []arr = { 1, -2, 1 }; 
    int k = 5; 
    Console.WriteLine(maxSubKSum(arr, k)); 
  
// This code is contributed by Ryuga

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PHP

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<?php 
  
// PHP implementation of the approach
  
// Function to concatenate array
function arrayConcatenate(&$arr, &$b, $k)
{
    // Array b will be formed by concatenation
    // array a exactly k times
    $j = 0;
    while ($k > 0) 
    {
  
        for ($i = 0; $i < sizeof($arr); $i++) 
        {
            $b[$j++] = $arr[$i];
        }
        $k--;
    }
}
  
// Function to return the maximum 
// subarray sum of arr[]
function maxSubArrSum(&$a)
{
    $size = sizeof($a);
    $max_so_far = 0;
    $max_ending_here = 0;
  
    for ($i = 0; $i < $size; $i++) 
    {
        $max_ending_here = $max_ending_here + $a[$i];
        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;
}
  
// Function to return the maximum sub-array 
// sum of the modified array
function maxSubKSum(&$arr,$k)
{
    $arrSum = 0;
    $maxSubArrSum = 0;
  
    $b = array_fill(0,(2 * sizeof($arr)),NULL);
  
    // Concatenating the array 2 times
    arrayConcatenate($arr, $b, 2);
  
    // Finding the sum of the array
    for ($i = 0; $i < sizeof($arr); $i++)
        $arrSum += $arr[$i];
  
    // If sum is less than zero
    if ($arrSum < 0)
        $maxSubArrSum = maxSubArrSum($b);
  
    // If sum is greater than zero
    else
        $maxSubArrSum = maxSubArrSum($b) +
                    ($k - 2) * $arrSum;
  
    return $maxSubArrSum;
}
  
    // Driver code
    $arr = array(1, -2, 1 );
    $k = 5;
    echo maxSubKSum($arr, $k);
      
// This code is contributed by Ita_c.   
?>

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Output:

2


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Improved By : Ryuga, AnupGaurav, Ita_c