Php Program to Find difference between sums of two diagonals

Given a matrix of n X n. The task is to calculate the absolute difference between the sums of its diagonal.
Examples:

Input : mat[][] = 11 2 4
                   4 5 6
                  10 8 -12 
Output : 15
Sum of primary diagonal = 11 + 5 + (-12) = 4.
Sum of primary diagonal = 4 + 5 + 10 = 19.
Difference = |19 - 4| = 15.


Input : mat[][] = 10 2
                   4 5
Output : 7

Calculate the sums across the two diagonals of a square matrix. Along the first diagonal of the matrix, row index = column index i.e mat[i][j] lies on the first diagonal if i = j. Along the other diagonal, row index = n – 1 – column index i.e mat[i][j] lies on the second diagonal if i = n-1-j. By using two loops we traverse the entire matrix and calculate the sum across the diagonals of the matrix.
Below is the implementation of this approach: 

PHP

<?php
// PHP program to find the difference
// between the sum of diagonal.

function difference($arr, $n)
{

    // Initialize sums of diagonals
    $d1 = 0; $d2 = 0;

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

            // finding sum of
            // primary diagonal
            if ($i == $j)
                $d1 += $arr[$i][$j];

            // finding sum of
            // secondary diagonal
            if ($i == $n - $j - 1)
                $d2 += $arr[$i][$j];
        }
    }

    // Absolute difference of the sums
    // across the diagonals
    return abs($d1 - $d2);
}

// Driver Code
{
    $n = 3;

    $arr = array(array(11, 2, 4),
                 array(4 , 5, 6),
                 array(10, 8, -12));

    echo difference($arr, $n);
    return 0;
}

// This code is contributed by nitin mittal.
?>

Output:  

15

Time Complexity: O(N*N), as we are using a loop to traverse N times.

Auxiliary Space: O(1), as we are not using any extra space.
We can optimize above solution to work in O(n) using the patterns present in indexes of cells. 

PHP

<?php
// PHP program to find the difference
// between the sum of diagonal.

function difference($arr, $n)
{

    // Initialize sums of diagonals
    $d1 = 0; $d2 = 0;

    for ($i = 0; $i < $n; $i++)
    {
        $d1 += $arr[$i][$i];
        $d2 += $arr[$i][$n-$i-1];
    }

    // Absolute difference of the sums
    // across the diagonals
    return abs($d1 - $d2);
}

// Driver Code
{
    $n = 3;

    $arr =array(array(11, 2, 4),
                array(4, 5, 6),
                array(10, 8, -12));

    echo difference($arr, $n);
    return 0;
}

// This code is contributed by nitin mittal.
?>

Output:  

15

Time Complexity: O(N), as we are using a loop to traverse N times.

Auxiliary Space: O(1), as we are not using any extra space.

Please refer complete article on Find difference between sums of two diagonals for more details!