Find maximum value of Sum( i*arr[i]) with only rotations on given array allowed

03 May, 2021
Given an array, only rotation operation is allowed on array. We can rotate the array as many times as we want. Return the maximum possible summation of i*arr[i].
Examples :  
Input: arr[] = {1, 20, 2, 10}
Output: 72
We can get 72 by rotating array twice.
{2, 10, 1, 20}
20*3 + 1*2 + 10*1 + 2*0 = 72

Input: arr[] = {10, 1, 2, 3, 4, 5, 6, 7, 8, 9}
Output: 330
We can get 330 by rotating array 9 times.
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
0*1 + 1*2 + 2*3 ... 9*10 = 330
We strongly recommend you to minimize your browser and try this yourself first.A Simple Solution is to find all rotations one by one, check sum of every rotation and return the maximum sum. Time complexity of this solution is O(n2). 
We can solve this problem in O(n) time using an Efficient Solution. Let Rj be value of i*arr[i] with j rotations. The idea is to calculate next rotation value from previous rotation, i.e., calculate Rj from Rj-1. We can calculate initial value of result as R0, then keep calculating next rotation values. 
How to efficiently calculate Rj from Rj-1? This can be done in O(1) time. Below are details.  
Let us calculate initial value of i*arr[i] with no rotation
R0 = 0*arr[0] + 1*arr[1] +...+ (n-1)*arr[n-1]

After 1 rotation arr[n-1], becomes first element of array, 
arr[0] becomes second element, arr[1] becomes third element
and so on.
R1 = 0*arr[n-1] + 1*arr[0] +...+ (n-1)*arr[n-2]

R1 - R0 = arr[0] + arr[1] + ... + arr[n-2] - (n-1)*arr[n-1]

After 2 rotations arr[n-2], becomes first element of array, 
arr[n-1] becomes second element, arr[0] becomes third element
and so on.
R2 = 0*arr[n-2] + 1*arr[n-1] +...+ (n-1)*arr[n-3]

R2 - R1 = arr[0] + arr[1] + ... + arr[n-3] - (n-1)*arr[n-2] + arr[n-1]

If we take a closer look at above values, we can observe 
below pattern

Rj - Rj-1 = arrSum - n * arr[n-j]

Where arrSum is sum of all array elements, i.e., 

arrSum = ∑ arr[i]
        0<=i<=n-1 
Below is complete algorithm: 
1) Compute sum of all array elements. Let this sum be 'arrSum'.

2) Compute R0 by doing i*arr[i] for given array. 
   Let this value be currVal.

3) Initialize result: maxVal = currVal // maxVal is result.

// This loop computes Rj from  Rj-1 
4) Do following for j = 1 to n-1
......a) currVal = currVal + arrSum-n*arr[n-j];
......b) If (currVal > maxVal)
            maxVal = currVal   

5) Return maxVal
Below is the implementation of above idea :
C++
Java
Python
C#
PHP
Javascript
// C++ program to find max value of i*arr[i]#include <iostream>using namespace std; // Returns max possible value of i*arr[i]int maxSum(int arr[], int n){    // Find array sum and i*arr[i] with no rotation    int arrSum = 0;  // Stores sum of arr[i]    int currVal = 0;  // Stores sum of i*arr[i]    for (int i=0; i<n; i++)    {        arrSum = arrSum + arr[i];        currVal = currVal+(i*arr[i]);    }     // Initialize result as 0 rotation sum    int maxVal = currVal;     // Try all rotations one by one and find    // the maximum rotation sum.    for (int j=1; j<n; j++)    {        currVal = currVal + arrSum-n*arr[n-j];        if (currVal > maxVal)            maxVal = currVal;    }     // Return result    return maxVal;} // Driver programint main(void){    int arr[] = {10, 1, 2, 3, 4, 5, 6, 7, 8, 9};    int n = sizeof(arr)/sizeof(arr[0]);    cout << "\nMax sum is " << maxSum(arr, n);    return 0;}
// Java program to find max value of i*arr[i] import java.util.Arrays; class Test{    static int arr[] = new int[]{10, 1, 2, 3, 4, 5, 6, 7, 8, 9};         // Returns max possible value of i*arr[i]    static int maxSum()    {        // Find array sum and i*arr[i] with no rotation        int arrSum = 0;  // Stores sum of arr[i]        int currVal = 0;  // Stores sum of i*arr[i]        for (int i=0; i<arr.length; i++)        {            arrSum = arrSum + arr[i];            currVal = currVal+(i*arr[i]);        }              // Initialize result as 0 rotation sum        int maxVal = currVal;              // Try all rotations one by one and find        // the maximum rotation sum.        for (int j=1; j<arr.length; j++)        {            currVal = currVal + arrSum-arr.length*arr[arr.length-j];            if (currVal > maxVal)                maxVal = currVal;        }              // Return result        return maxVal;    }         // Driver method to test the above function    public static void main(String[] args)    {        System.out.println("Max sum is " + maxSum());    }}
'''Python program to find maximum value of Sum(i*arr[i])''' # returns max possible value of Sum(i*arr[i])def maxSum(arr):     # stores sum of arr[i]    arrSum = 0        # stores sum of i*arr[i]    currVal = 0         n = len(arr)     for i in range(0, n):        arrSum = arrSum + arr[i]        currVal = currVal + (i*arr[i])     # initialize result    maxVal = currVal     # try all rotations one by one and find the maximum    # rotation sum    for j in range(1, n):        currVal = currVal + arrSum-n*arr[n-j]        if currVal > maxVal:            maxVal = currVal     # return result    return maxVal # test maxsum(arr) functionarr = [10, 1, 2, 3, 4, 5, 6, 7, 8, 9]print "Max sum is: ", maxSum(arr)
// C# program to find max value of i*arr[i]using System; class Test{    static int []arr = new int[]{10, 1, 2, 3, 4,                                  5, 6, 7, 8, 9};         // Returns max possible value of i*arr[i]    static int maxSum()    {        // Find array sum and i*arr[i]        // with no rotation        int arrSum = 0; // Stores sum of arr[i]        int currVal = 0; // Stores sum of i*arr[i]                 for (int i = 0; i < arr.Length; i++)        {            arrSum = arrSum + arr[i];            currVal = currVal + (i * arr[i]);        }             // Initialize result as 0 rotation sum        int maxVal = currVal;             // Try all rotations one by one and find        // the maximum rotation sum.        for (int j = 1; j < arr.Length; j++)        {            currVal = currVal + arrSum - arr.Length *                                arr[arr.Length - j];            if (currVal > maxVal)                maxVal = currVal;        }             // Return result        return maxVal;    }         // Driver Code    public static void Main()    {        Console.WriteLine("Max sum is " + maxSum());    }} // This article is contributed by vt_m.
<?php// PHP program to find max// value of i*arr[i] // Returns max possible// value of i*arr[i]function maxSum($arr, $n){    // Find array sum and    // i*arr[i] with no rotation         // Stores sum of arr[i]    $arrSum = 0;         // Stores sum of i*arr[i]    $currVal = 0;    for ($i = 0; $i < $n; $i++)    {        $arrSum = $arrSum + $arr[$i];        $currVal = $currVal +                  ($i * $arr[$i]);    }     // Initialize result as    // 0 rotation sum    $maxVal = $currVal;     // Try all rotations one    // by one and find the    // maximum rotation sum.    for ($j = 1; $j < $n; $j++)    {        $currVal = $currVal + $arrSum -                   $n * $arr[$n - $j];        if ($currVal > $maxVal)            $maxVal = $currVal;    }     // Return result    return $maxVal;} // Driver Code$arr = array (10, 1, 2, 3, 4,              5, 6, 7, 8, 9);$n = sizeof($arr);echo "Max sum is " ,     maxSum($arr, $n); // This code is contributed by m_kit?>
<script>// JavaScript program to find max value of i*arr[i] // Returns max possible value of i*arr[i]function maxSum(arr, n){    // Find array sum and i*arr[i] with no rotation    let arrSum = 0; // Stores sum of arr[i]    let currVal = 0; // Stores sum of i*arr[i]    for (let i=0; i<n; i++)    {        arrSum = arrSum + arr[i];        currVal = currVal+(i*arr[i]);    }     // Initialize result as 0 rotation sum    let maxVal = currVal;     // Try all rotations one by one and find    // the maximum rotation sum.    for (let j=1; j<n; j++)    {        currVal = currVal + arrSum-n*arr[n-j];        if (currVal > maxVal)            maxVal = currVal;    }     // Return result    return maxVal;} // Driver program    let arr = [10, 1, 2, 3, 4, 5, 6, 7, 8, 9];    let n = arr.length;    document.write("Max sum is " + maxSum(arr, n));      // This code is contributed by Surbhi Tyagi.</script>
Output : 
Max sum is 330
Time Complexity : O(n) Auxiliary Space : O(1)
This article is contributed by Nitesh Singh. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above 
jit_t
Praveen Sinha
SohamGidwani
13manujchaturvedi
surbhityagi15
rotation
Arrays
Arrays
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.