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 = 9Input: 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 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.
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Below is the implementation of the above approach:
C++
// C++ implementation of the approach #include<bits/stdc++.h> using namespace std; // Function to concatenate array void arrayConcatenate(int *arr, int *b, int k,int len) { // Array b will be formed by concatenation // array a exactly k times int j = 0; while (k > 0) { for (int i = 0; i < len; i++) { b[j++] = arr[i]; } k--; } } // Function to return the maximum // subarray sum of arr[] long maxSubArrSum(int *a,int len) { int size = len; int max_so_far = INT_MIN; long 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 long maxSubKSum(int *arr, int k,int len) { int arrSum = 0; long maxSubArrSum1 = 0; int b[(2 * len)]={0}; // Concatenating the array 2 times arrayConcatenate(arr, b, 2,len); // Finding the sum of the array for (int i = 0; i < len; i++) arrSum += arr[i]; // If sum is less than zero if (arrSum < 0) maxSubArrSum1 = maxSubArrSum(b,2*len); // If sum is greater than zero else maxSubArrSum1 = maxSubArrSum(b,2*len) + (k - 2) * arrSum; return maxSubArrSum1; } // Driver code int main() { int arr[] = { 1, -2, 1 }; int arrlen=sizeof(arr)/sizeof(arr[0]); int k = 5; cout << maxSubKSum(arr, k,arrlen) << endl; return 0; } // This code is contributed by mits |
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Java
// 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
# 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#
// 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
<?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
