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Maximum sum of pairwise product in an array with negative allowed
• Difficulty Level : Easy
• Last Updated : 14 Apr, 2021

Given an array of n elements. Find maximum sum of pairwise multiplications. Sum can be larger so take mod with 10^9+7. If there are odd elements, then we can add any one element (without forming a pair) to the sum.
Examples:

```Input : arr[] = {-1, 4, 5, -7, -4, 9, 0}
Output : 77
So to get the maximum sum, the arrangement will
be {-7, -4}, {-1, 0}, {9, 5} and {4}.
So the answer is (-7*(-4))+((-1)*0)+(9*5)+(4) ={77}.

Input : arr[] = {8, 7, 9}
Output : 79
Answer is (9*8) +(7) = 79.```

1- Sort the given array.
2- First, multiply the negative numbers pairwise from the starting and add to the total_sum.
3- Second, multiply the positive numbers pairwise from the last and to the total_sum.
4- Check if negative and positive both counts are odd, then add the product of last pair
i.e. last negative and positive left.
5- Or if any of the one counts is odd, then add that element left.
6- Return sum.

## C++

 `// C++ program for above implementation``#include ``#define Mod 1000000007``using` `namespace` `std;` `// Function to find the maximum sum``long` `long` `int` `findSum(``int` `arr[], ``int` `n)``{``    ``long` `long` `int` `sum = 0;` `    ``// Sort the array first``    ``sort(arr, arr + n);` `    ``// First multiply negative numbers pairwise``    ``// and sum up from starting as to get maximum``    ``// sum.``    ``int` `i = 0;``    ``while` `(i < n && arr[i] < 0) {``        ``if` `(i != n - 1 && arr[i + 1] <= 0) {``            ``sum = (sum + (arr[i] * arr[i + 1]) % Mod) % Mod;``            ``i += 2;``        ``}``        ``else``            ``break``;``    ``}` `    ``// Second multiply positive numbers pairwise``    ``// and summed up from the last as to get maximum``    ``// sum.``    ``int` `j = n - 1;``    ``while` `(j >= 0 && arr[j] > 0) {``        ``if` `(j != 0 && arr[j - 1] > 0) {``            ``sum = (sum + (arr[j] * arr[j - 1]) % Mod) % Mod;``            ``j -= 2;``        ``}``        ``else``            ``break``;``    ``}` `    ``// To handle case if positive and negative``    ``// numbers both are odd in counts.``    ``if` `(j > i)``        ``sum = (sum + (arr[i] * arr[j]) % Mod) % Mod;` `    ``// If one of them occurs odd times``    ``else` `if` `(i == j)``        ``sum = (sum + arr[i]) % Mod;` `    ``return` `sum;``}` `// Drivers code``int` `main()``{``    ``int` `arr[] = { -1, 9, 4, 5, -4, 7 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``cout << findSum(arr, n);``    ``return` `0;``}`

## Java

 `// Java program for above implementation``import` `java.io.*;``import` `java.util.*;` `class` `GFG {` `static` `int` `Mod = ``1000000007``;` `// Function to find the maximum sum``static` `long` `findSum(``int` `arr[], ``int` `n) {``    ``long` `sum = ``0``;` `    ``// Sort the array first``    ``Arrays.sort(arr);` `    ``// First multiply negative numbers``    ``// pairwise and sum up from starting``    ``// as to get maximum sum.``    ``int` `i = ``0``;``    ``while` `(i < n && arr[i] < ``0``) {``    ``if` `(i != n - ``1` `&& arr[i + ``1``] <= ``0``) {``        ``sum = (sum + (arr[i] * arr[i + ``1``]) % Mod) % Mod;``        ``i += ``2``;``    ``}``    ``else``        ``break``;``    ``}` `    ``// Second multiply positive numbers``    ``// pairwise and summed up from the``    ``// last as to get maximum sum.``    ``int` `j = n - ``1``;``    ``while` `(j >= ``0` `&& arr[j] > ``0``) {``    ``if` `(j != ``0` `&& arr[j - ``1``] > ``0``) {``        ``sum = (sum + (arr[j] * arr[j - ``1``]) % Mod) % Mod;``        ``j -= ``2``;``    ``} ``else``        ``break``;``    ``}` `    ``// To handle case if positive and negative``    ``// numbers both are odd in counts.``    ``if` `(j > i)``    ``sum = (sum + (arr[i] * arr[j]) % Mod) % Mod;` `    ``// If one of them occurs odd times``    ``else` `if` `(i == j)``    ``sum = (sum + arr[i]) % Mod;` `    ``return` `sum;``}` `// Drivers code``public` `static` `void` `main(String args[]) {``    ``int` `arr[] = {-``1``, ``9``, ``4``, ``5``, -``4``, ``7``};``    ``int` `n = arr.length;``    ``System.out.println(findSum(arr, n));``}``}` `/*This code is contributed by Nikita Tiwari.*/`

## Python3

 `# Python3 code for above implementation``Mod``=` `1000000007` `# Function to find the maximum sum``def` `findSum(arr, n):``    ``sum` `=` `0``    ` `    ``# Sort the array first``    ``arr.sort()``    ` `    ``# First multiply negative numbers``    ``# pairwise and sum up from starting``    ``# as to get maximum sum.``    ``i ``=` `0``    ``while` `i < n ``and` `arr[i] < ``0``:``        ``if` `i !``=` `n ``-` `1` `and` `arr[i ``+` `1``] <``=` `0``:``            ``sum` `=` `(``sum` `+` `(arr[i] ``*` `arr[i ``+` `1``])``                                 ``%` `Mod) ``%` `Mod``            ``i ``+``=` `2``        ``else``:``            ``break``        ` `    ``# Second multiply positive numbers``    ``# pairwise and summed up from the``    ``# last as to get maximum sum.``    ``j ``=` `n ``-` `1``    ``while` `j >``=` `0` `and` `arr[j] > ``0``:``        ``if` `j !``=` `0` `and` `arr[j ``-` `1``] > ``0``:``            ``sum` `=` `(``sum` `+` `(arr[j] ``*` `arr[j ``-` `1``])``                                 ``%` `Mod) ``%` `Mod``            ``j ``-``=` `2``        ``else``:``            ``break``        ` `    ``# To handle case if positive``    ``# and negative numbers both``    ``# are odd in counts.``    ``if` `j > i:``        ``sum` `=` `(``sum` `+` `(arr[i] ``*` `arr[j]) ``%` `Mod)``                                       ``%` `Mod``        ` `    ``# If one of them occurs odd times``    ``elif` `i ``=``=` `j:``        ``sum` `=` `(``sum` `+` `arr[i]) ``%` `Mod``    ` `    ``return` `sum` `# Driver code``arr ``=` `[ ``-``1``, ``9``, ``4``, ``5``, ``-``4``, ``7` `]``n ``=` `len``(arr)``print``(findSum(arr, n))` `# This code is contributed by "Sharad_Bhardwaj".`

## C#

 `// C# program for above implementation``using` `System;` `class` `GFG {` `    ``static` `int` `Mod = 1000000007;` `    ``// Function to find the maximum sum``    ``static` `long` `findSum(``int``[] arr, ``int` `n)``    ``{``        ``long` `sum = 0;` `        ``// Sort the array first``        ``Array.Sort(arr);` `        ``// First multiply negative numbers``        ``// pairwise and sum up from starting``        ``// as to get maximum sum.``        ``int` `i = 0;``        ``while` `(i < n && arr[i] < 0) {``            ``if` `(i != n - 1 && arr[i + 1] <= 0) {``                ``sum = (sum + (arr[i] * arr[i + 1]) % Mod) % Mod;``                ``i += 2;``            ``}``            ``else``                ``break``;``        ``}` `        ``// Second multiply positive numbers``        ``// pairwise and summed up from the``        ``// last as to get maximum sum.``        ``int` `j = n - 1;``        ``while` `(j >= 0 && arr[j] > 0) {``            ``if` `(j != 0 && arr[j - 1] > 0) {``                ``sum = (sum + (arr[j] * arr[j - 1]) % Mod) % Mod;``                ``j -= 2;``            ``}``            ``else``                ``break``;``        ``}` `        ``// To handle case if positive and negative``        ``// numbers both are odd in counts.``        ``if` `(j > i)``            ``sum = (sum + (arr[i] * arr[j]) % Mod) % Mod;` `        ``// If one of them occurs odd times``        ``else` `if` `(i == j)``            ``sum = (sum + arr[i]) % Mod;` `        ``return` `sum;``    ``}` `    ``// Drivers code``    ``public` `static` `void` `Main()``    ``{``        ``int``[] arr = { -1, 9, 4, 5, -4, 7 };``        ``int` `n = arr.Length;``        ``Console.WriteLine(findSum(arr, n));``    ``}``}` `/*This code is contributed by vt_m.*/`

## PHP

 `= 0 && ``\$arr``[``\$j``] > 0)``    ``{``        ``if` `(``\$j` `!= 0 && ``\$arr``[``\$j` `- 1] > 0)``        ``{``            ``\$sum` `= (``\$sum` `+ (``\$arr``[``\$j``] *``                    ``\$arr``[``\$j` `- 1]) % ``\$Mod``) % ``\$Mod``;``            ``\$j` `-= 2;``        ``}``        ``else``            ``break``;``    ``}` `    ``// To handle case if positive and negative``    ``// numbers both are odd in counts.``    ``if` `(``\$j` `> ``\$i``)``        ``\$sum` `= (``\$sum` `+ (``\$arr``[``\$i``] *``                ``\$arr``[``\$j``]) % ``\$Mod``) % ``\$Mod``;` `    ``// If one of them occurs odd times``    ``else` `if` `(``\$i` `== ``\$j``)``        ``\$sum` `= (``\$sum` `+ ``\$arr``[``\$i``]) % Mod;` `    ``return` `\$sum``;``}` `// Driver code``\$arr` `= ``array` `(-1, 9, 4, 5, -4, 7 );``\$n` `= sizeof(``\$arr``);``echo` `findSum(``\$arr``, ``\$n``);` `// This code is contributed by ita_c``?>`

## Javascript

 ``

Output:

`87`

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