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Sum of maximum of all subarrays | Divide and Conquer
• Difficulty Level : Hard
• Last Updated : 09 May, 2020

Given an array arr[] of length N, the task is to find the sum of the maximum elements of every possible sub-array of the array.

Examples:

```Input : arr[] = {1, 3, 1, 7}
Output : 42
Max of all sub-arrays:
{1} - 1
{1, 3} - 3
{1, 3, 1} - 3
{1, 3, 1, 7} - 7
{3} - 3
{3, 1} - 3
{3, 1, 7} - 7
{1} - 1
{1, 7} - 7
{7} - 7
1 + 3 + 3 + 7 + 3 + 3 + 7 + 1 + 7 + 7 = 42

Input : arr[] = {1, 1, 1, 1, 1}
Output : 15
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach :
In this article, we will learn how to solve this problem using divide and conquer.
Let’s assume that element at ith index is largest of all. For any sub-array that contains index ‘i’, the element at ‘i’ will always be maximum in the sub-array.
If element at ith index is largest, we can safely say, that element ith index will be largest in (i+1)*(N-i) subarrays. So, its total contribution will be arr[i]*(i+1)*(N-i). Now, we will divide the array in two parts, (0, i-1) and (i+1, N-1) and apply the same algorithms to both of them separately.

So our general recurrence relation will be:

```maxSumSubarray(arr, l, r) = arr[i]*(r-i+1)*(i-l+1)
+ maxSumSubarray(arr, l, i-1)
+ maxSumSubarray(arr, i+1, r)
where i is index of maximum element in range [l, r].
```

Now, we need a way to efficiently answer rangeMax() queries. Segment tree will be an efficient way to answer this query. We will need to answer this query at most N times. Thus, the time complexity of our divide and conquer algorithm will O(Nlog(N)).
If we have to answer the problem “Sum of minimum of all subarrays” then we will use the segment tree to answer rangeMin() queries. For this, you can go through the article segment tree range minimum.

Below is the implementation code:

## C++

 `// C++ implementation of the above approach`` ` `#include ``#define seg_max 51``using` `namespace` `std;`` ` `// Array to store segment tree.``// In first we will store the maximum``// of a range``// In second, we will store index of``// that range``pair<``int``, ``int``> seg_tree[seg_max];`` ` `// Size of array declared global``// to maintain simplicity in code``int` `n;`` ` `// Function to build segment tree``pair<``int``, ``int``> buildMaxTree(``int` `l, ``int` `r, ``int` `i, ``int` `arr[])``{``    ``// Base case``    ``if` `(l == r) {``        ``seg_tree[i] = { arr[l], l };``        ``return` `seg_tree[i];``    ``}`` ` `    ``// Finding the maximum among left and right child``    ``seg_tree[i] = max(buildMaxTree(l, (l + r) / 2, 2 * i + 1, arr),``                      ``buildMaxTree((l + r) / 2 + 1, r, 2 * i + 2, arr));`` ` `    ``// Returning the maximum to parent``    ``return` `seg_tree[i];``}`` ` `// Function to perform range-max query in segment tree``pair<``int``, ``int``> rangeMax(``int` `l, ``int` `r, ``int` `arr[],``                        ``int` `i = 0, ``int` `sl = 0, ``int` `sr = n - 1)``{``    ``// Base cases``    ``if` `(sr < l || sl > r)``        ``return` `{ INT_MIN, -1 };``    ``if` `(sl >= l and sr <= r)``        ``return` `seg_tree[i];`` ` `    ``// Finding the maximum among left and right child``    ``return` `max(rangeMax(l, r, arr, 2 * i + 1, sl, (sl + sr) / 2),``               ``rangeMax(l, r, arr, 2 * i + 2, (sl + sr) / 2 + 1, sr));``}`` ` `// Function to find maximum sum subarray``int` `maxSumSubarray(``int` `arr[], ``int` `l = 0, ``int` `r = n - 1)``{``    ``// base case``    ``if` `(l > r)``        ``return` `0;`` ` `    ``// range-max query to determine``    ``// largest in the range.``    ``pair<``int``, ``int``> a = rangeMax(l, r, arr);`` ` `    ``// divide the array in two parts``    ``return` `a.first * (r - a.second + 1) * (a.second - l + 1)``           ``+ maxSumSubarray(arr, l, a.second - 1)``           ``+ maxSumSubarray(arr, a.second + 1, r);``}`` ` `// Driver Code``int` `main()``{``    ``// Input array``    ``int` `arr[] = { 1, 3, 1, 7 };`` ` `    ``// Size of array``    ``n = ``sizeof``(arr) / ``sizeof``(``int``);`` ` `    ``// Builind the segment-tree``    ``buildMaxTree(0, n - 1, 0, arr);`` ` `    ``cout << maxSumSubarray(arr);`` ` `    ``return` `0;``}`

## Java

 `// Java implementation of the above approach``class` `GFG {``    ``static` `class` `pair {``        ``int` `first, second;`` ` `        ``public` `pair(``int` `first, ``int` `second) {``            ``this``.first = first;``            ``this``.second = second;``        ``}``    ``}`` ` `    ``static` `final` `int` `seg_max = ``51``;``     ` `    ``// Array to store segment tree.``    ``// In first we will store the maximum``    ``// of a range``    ``// In second, we will store index of``    ``// that range``    ``static` `pair[] seg_tree = ``new` `pair[seg_max];`` ` `    ``// Size of array declared global``    ``// to maintain simplicity in code``    ``static` `int` `n;`` ` `    ``// Function to build segment tree``    ``static` `pair buildMaxTree(``int` `l, ``int` `r, ``int` `i, ``int` `arr[]) ``    ``{``        ``// Base case``        ``if` `(l == r) {``            ``seg_tree[i] = ``new` `pair(arr[l], l);``            ``return` `seg_tree[i];``        ``}`` ` `        ``// Finding the maximum among left and right child``        ``seg_tree[i] = max(buildMaxTree(l, (l + r) / ``2``, ``2` `* i + ``1``, arr),``                ``buildMaxTree((l + r) / ``2` `+ ``1``, r, ``2` `* i + ``2``, arr));`` ` `        ``// Returning the maximum to parent``        ``return` `seg_tree[i];``    ``}`` ` `    ``// Function to perform range-max query in segment tree``    ``static` `pair rangeMax(``int` `l, ``int` `r, ``int` `arr[], ``                        ``int` `i, ``int` `sl, ``int` `sr)``    ``{``        ``// Base cases``        ``if` `(sr < l || sl > r)``            ``return` `new` `pair(Integer.MIN_VALUE, -``1``);``        ``if` `(sl >= l && sr <= r)``            ``return` `seg_tree[i];`` ` `        ``// Finding the maximum among left and right child``        ``return` `max(rangeMax(l, r, arr, ``2` `* i + ``1``, sl, (sl + sr) / ``2``),``                ``rangeMax(l, r, arr, ``2` `* i + ``2``, (sl + sr) / ``2` `+ ``1``, sr));``    ``}`` ` `    ``static` `pair max(pair f, pair s) {``        ``if` `(f.first > s.first)``            ``return` `f;``        ``else``            ``return` `s;``    ``}`` ` `    ``// Function to find maximum sum subarray``    ``static` `int` `maxSumSubarray(``int` `arr[], ``int` `l, ``int` `r)``    ``{``        ``// base case``        ``if` `(l > r)``            ``return` `0``;`` ` `        ``// range-max query to determine``        ``// largest in the range.``        ``pair a = rangeMax(l, r, arr, ``0``, ``0``, n - ``1``);`` ` `        ``// divide the array in two parts``        ``return` `a.first * (r - a.second + ``1``) * (a.second - l + ``1``) ``                ``+ maxSumSubarray(arr, l, a.second - ``1``)``                ``+ maxSumSubarray(arr, a.second + ``1``, r);``    ``}`` ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``// Input array``        ``int` `arr[] = { ``1``, ``3``, ``1``, ``7` `};`` ` `        ``// Size of array``        ``n = arr.length;`` ` `        ``// Builind the segment-tree``        ``buildMaxTree(``0``, n - ``1``, ``0``, arr);`` ` `        ``System.out.print(maxSumSubarray(arr, ``0``, n - ``1``));``    ``}``}`` ` `// This code is contributed by 29AjayKumar`

## C#

 `// C# implementation of the above approach``using` `System;`` ` `class` `GFG {``    ``class` `pair {``        ``public` `int` `first, second;``  ` `        ``public` `pair(``int` `first, ``int` `second) {``            ``this``.first = first;``            ``this``.second = second;``        ``}``    ``}``  ` `    ``static` `readonly` `int` `seg_max = 51;``      ` `    ``// Array to store segment tree.``    ``// In first we will store the maximum``    ``// of a range``    ``// In second, we will store index of``    ``// that range``    ``static` `pair[] seg_tree = ``new` `pair[seg_max];``  ` `    ``// Size of array declared global``    ``// to maintain simplicity in code``    ``static` `int` `n;``  ` `    ``// Function to build segment tree``    ``static` `pair buildMaxTree(``int` `l, ``int` `r, ``int` `i, ``int` `[]arr) ``    ``{``        ``// Base case``        ``if` `(l == r) {``            ``seg_tree[i] = ``new` `pair(arr[l], l);``            ``return` `seg_tree[i];``        ``}``  ` `        ``// Finding the maximum among left and right child``        ``seg_tree[i] = max(buildMaxTree(l, (l + r) / 2, 2 * i + 1, arr),``                ``buildMaxTree((l + r) / 2 + 1, r, 2 * i + 2, arr));``  ` `        ``// Returning the maximum to parent``        ``return` `seg_tree[i];``    ``}``  ` `    ``// Function to perform range-max query in segment tree``    ``static` `pair rangeMax(``int` `l, ``int` `r, ``int` `[]arr, ``                        ``int` `i, ``int` `sl, ``int` `sr)``    ``{``        ``// Base cases``        ``if` `(sr < l || sl > r)``            ``return` `new` `pair(``int``.MinValue, -1);``        ``if` `(sl >= l && sr <= r)``            ``return` `seg_tree[i];``  ` `        ``// Finding the maximum among left and right child``        ``return` `max(rangeMax(l, r, arr, 2 * i + 1, sl, (sl + sr) / 2),``                ``rangeMax(l, r, arr, 2 * i + 2, (sl + sr) / 2 + 1, sr));``    ``}``  ` `    ``static` `pair max(pair f, pair s) {``        ``if` `(f.first > s.first)``            ``return` `f;``        ``else``            ``return` `s;``    ``}``  ` `    ``// Function to find maximum sum subarray``    ``static` `int` `maxSumSubarray(``int` `[]arr, ``int` `l, ``int` `r)``    ``{``        ``// base case``        ``if` `(l > r)``            ``return` `0;``  ` `        ``// range-max query to determine``        ``// largest in the range.``        ``pair a = rangeMax(l, r, arr, 0, 0, n - 1);``  ` `        ``// divide the array in two parts``        ``return` `a.first * (r - a.second + 1) * (a.second - l + 1) ``                ``+ maxSumSubarray(arr, l, a.second - 1)``                ``+ maxSumSubarray(arr, a.second + 1, r);``    ``}``  ` `    ``// Driver Code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``// Input array``        ``int` `[]arr = { 1, 3, 1, 7 };``  ` `        ``// Size of array``        ``n = arr.Length;``  ` `        ``// Builind the segment-tree``        ``buildMaxTree(0, n - 1, 0, arr);``  ` `        ``Console.Write(maxSumSubarray(arr, 0, n - 1));``    ``}``}`` ` `// This code is contributed by PrinciRaj1992`
Output:
```42
```

Time complexity : O(Nlog(N))

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