Sum of Areas of Rectangles possible for an array

Given an array, task is to compute the sum of all possible maximum area rectangle which can be formed from the array elements. Also, you can reduce the elements of the array by at most 1.

Examples :

Input :a = {10, 10, 10, 10, 11, 
            10, 11, 10}
Output : 210
Explanation : 
We can form two rectangles one square (10 * 10) 
and one (11 * 10). Hence, total area = 100 + 110 = 210.

Input : a = { 3, 4, 5, 6 }
Output : 15
Explanation : 
We can reduce 4 to 3 and 6 to 5 so that we got 
rectangle of (3 * 5). Hence area = 15.

Input : a = { 3, 2, 5, 2 }
Output : 0

Naive Approach : Check for all possible four elements of the array and then whichever can form a rectangle. In these rectangles, separate all those rectangles which are of maximum area formed by these elements. After getting the rectangles and their areas, sum them all to get our desired solution.

Efficient Approach : To get the maximum area rectangle, first sort the elements of the array in non-increasing array. After sorting, start the procedure to select the elements of the array. Here, selection of two elements of array (as length of rectangle) is possible if elements of array are equal (a[i] == a[i+1]) or if length of smaller element a[i+1] is one less than a[i] (in this case we have our length a[i+1] because a[i] is decreased by 1). One flag variable is maintained to check that whether we get length and breadth both. After getting length, set the flag variable, now calculate the breadth in the same way as we have done for length and sum the area of rectangle. After getting length and breadth both, again set the flag variable false so that we will now search for a new rectangle. This process is repeated and lastly, final sum of area is returned.

C++


[sourcecode language=”CPP”]
// CPP code to find sum of all
// area rectangle possible
#include <bits/stdc++.h>
using namespace std;

// Function to find
// area of rectangles
int MaxTotalRectangleArea(int a[],
int n)
{
// sorting the array in
// descending order
sort(a, a + n, greater<int>());

// store the final sum of
// all the rectangles area
// possible
int sum = 0;
bool flag = false;

// temporary variable to store
// the length of rectangle
int len;

for (int i = 0; i < n; i++)
{

// Selecting the length of
// rectangle so that difference
// between any two number is 1
// only. Here length is selected
// so flag is set
if ((a[i] == a[i + 1] || a[i] –
a[i + 1] == 1) && (!flag))
{
// flag is set means
// we have got length of
// rectangle
flag = true;

// length is set to
// a[i+1] so that if
// a[i] a[i+1] is less
// than by 1 then also
// we have the correct
// choice for length
len = a[i + 1];

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
i++;
}

// Selecting the width of rectangle
// so that difference between any
// two number is 1 only. Here width
// is selected so now flag is again
// unset for next rectangle
else if ((a[i] == a[i + 1] ||
a[i] – a[i + 1] == 1) && (flag))
{
// area is calculated for
// rectangle
sum = sum + a[i + 1] * len;

// flag is set false
// for another rectangle
// which we can get from
// elements in array
flag = false;

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
i++;
}
}

return sum;
}

// Driver code
int main()
{
int a[] = { 10, 10, 10, 10,
11, 10, 11, 10,
9, 9, 8, 8 };
int n = sizeof(a) / sizeof(a[0]);

cout << MaxTotalRectangleArea(a, n);

return 0;
}

[/sourcecode]

C#


[sourcecode language=”csharp”]
// C# code to find sum of all area rectangle
// possible
using System;

class GFG {

// Function to find
// area of rectangles
static int MaxTotalRectangleArea(int []a,
int n)
{

// sorting the array in descending
// order
Array.Sort(a);

// store the final sum of all the
// rectangles area possible
int sum = 0;
bool flag = false;

// temporary variable to store the
// length of rectangle
int len =0;

for (int i = 0; i < n; i++)
{

// Selecting the length of
// rectangle so that difference
// between any two number is 1
// only. Here length is selected
// so flag is set
if ((a[i] == a[i + 1] || a[i] –
a[i + 1] == 1) && (!flag))
{
// flag is set means
// we have got length of
// rectangle
flag = true;

// length is set to
// a[i+1] so that if
// a[i] a[i+1] is less
// than by 1 then also
// we have the correct
// choice for length
len = a[i + 1];

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
i++;
}

// Selecting the width of rectangle
// so that difference between any
// two number is 1 only. Here width
// is selected so now flag is again
// unset for next rectangle
else if ((a[i] == a[i + 1] ||
a[i] – a[i + 1] == 1) && (flag))
{
// area is calculated for
// rectangle
sum = sum + a[i + 1] * len;

// flag is set false
// for another rectangle
// which we can get from
// elements in array
flag = false;

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
i++;
}
}

return sum;
}

// Driver code
static public void Main ()
{
int []a = { 10, 10, 10, 10,
11, 10, 11, 10,
9, 9, 8, 8 };
int n = a.Length;

Console.WriteLine(
MaxTotalRectangleArea(a, n));
}
}

// This code is contributed by anuj_67.
[/sourcecode]

PHP


[sourcecode language=”PHP”]
<?php
// PHP code to find sum
// of all area rectangle
// possible

// Function to find
// area of rectangles
function MaxTotalRectangleArea( $a, $n)
{
// sorting the array in
// descending order
rsort($a);

// store the final sum of
// all the rectangles area
// possible
$sum = 0;
$flag = false;

// temporary variable to store
// the length of rectangle
$len;

for ( $i = 0; $i < $n; $i++)
{

// Selecting the length of
// rectangle so that difference
// between any two number is 1
// only. Here length is selected
// so flag is set
if (($a[$i] == $a[$i + 1] or $a[$i] –
$a[$i + 1] == 1) and (!$flag))
{
// flag is set means
// we have got length of
// rectangle
$flag = true;

// length is set to
// a[i+1] so that if
// a[i+1] is less than a[i]
// by 1 then also we have
// the correct chice for length
$len = $a[$i + 1];

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
$i++;
}

// Selecting the width of rectangle
// so that difference between any
// two number is 1 only. Here width
// is selected so now flag is again
// unset for next rectangle
else if (($a[$i] == $a[$i + 1] or
$a[$i] – $a[$i + 1] == 1) and
($flag))
{
// area is calculated for
// rectangle
$sum = $sum + $a[$i + 1] * $len;

// flag is set false
// for another rectangle
// which we can get from
// elements in array
$flag = false;

// incrementing the counter
// one time more as we have
// considered a[i+1] element
// also so.
$i++;
}
}

return $sum;
}

// Driver code
$a = array( 10, 10, 10, 10,
11, 10, 11, 10,
9, 9, 8, 8 );
$n = count($a);

echo MaxTotalRectangleArea($a, $n);

//This code is contributed by anuj_67.
?>

[/sourcecode]

Output :

282

Time Complexity : O(nlog(n))
Auxiliary Space : O(1)



My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.