Problem Statement

Given a square matrix (2D array), calculate the sum of its two diagonals.

The two diagonals in consideration are the primary diagonal that spans from the top-left to the bottom-right and the secondary diagonal that spans from top-right to bottom-left. If a number is part of both diagonals (which occurs only for odd-sized matrices), it should be counted only once in the sum.

Examples

1. Example 1:
   • Input:

     [[1,2,3],
      [4,5,6],
      [7,8,9]]
     

   • Expected Output: 25
   • Justification: Summing up the two diagonals (1+5+9+3+7), we get 25. Please note that the element at [1][1] = 5 is counted only once.
2. Example 2:
   • Input:

     [[1,0],
      [0,1]]
     

   • Expected Output: 2
   • Justification: The sum of the two diagonals is 1+1 = 2.
3. Example 3:
   • Input:

     [[5]]
     

   • Expected Output: 5
   • Justification: Since there's only one element, it is the sum itself.

Constraints:

• n == mat.length == mat[i].length
• 1 <= n <= 100
• 1 <= mat[i][j] <= 100

Solution

The primary algorithmic approach for this problem is straightforward yet efficient, involving a single loop traversal through the matrix, accumulating the sum of the diagonal elements as it progresses. This approach is effective in that it avoids unnecessary calculations or traversals through the matrix, adhering strictly to the diagonal indices to retrieve the values to be summed. The main mechanism involves identifying and summing the elements of the primary and secondary diagonal, ensuring that, if the matrix size is odd, the central element (which belongs to both diagonals) is not double-counted.

Step-by-Step Agorithm

1. Initialize Sum: Start by initializing a variable to store the sum of the diagonal elements. Let's call this variable diagonalSum.

2. Loop Through Matrix: Iterate through the matrix using a loop. Since the matrix is square, you can use a single index to traverse both rows and columns. Let's use i as the loop variable, ranging from 0 to the length of the matrix minus one.

3. Add Primary Diagonal Elements: In each iteration, add the element at the primary diagonal to diagonalSum. The primary diagonal elements are those where the row and column indices are equal, i.e., matrix[i][i].

4. Add Secondary Diagonal Elements: In the same iteration, add the element at the secondary diagonal to diagonalSum. The secondary diagonal elements are those where the column index is the complement of the row index, i.e., matrix[i][matrix.length - 1 - i].

5. Avoid Double Counting: If the matrix has an odd number of rows and columns, the central element will be counted twice (once for each diagonal). To correct this, subtract the central element from diagonalSum. The central element is at the position matrix[middle][middle], where middle = matrix.length / 2.

6. Return the Result: After completing the loop, return the value of diagonalSum. This is the sum of the elements on both diagonals of the matrix.

Algorithm Walkthrough