/**
* Returns the sum of the corner elements of a matrix.
*
* Examples:
*
* 1 2 3 4
* 5 6 7 8
* 9 10 11 12
*
* Corners:
* - 1 (top-left)
* - 4 (top-right)
* - 9 (bottom-left)
* - 12 (bottom-right)
*
* Sum = 26
*
* This implementation counts each corner position only once,
* so it correctly handles:
* - 1x1 matrices
* - 1xN matrices
* - Nx1 matrices
*
* @param matrix A rectangular matrix represented as a list of lists.
* @return The sum of the unique corner elements.
* @throws IllegalArgumentException if the matrix is empty.
*/
fun sumCorners(matrix: List<List<Int>>): Int {
require(matrix.isNotEmpty()) { "Matrix must not be empty." }
require(matrix[0].isNotEmpty()) { "Matrix rows must not be empty." }
val rowCount = matrix.size
val columnCount = matrix[0].size
// Store corner coordinates.
val corners = setOf(
0 to 0, // Top-left
0 to (columnCount - 1), // Top-right
(rowCount - 1) to 0, // Bottom-left
(rowCount - 1) to (columnCount - 1) // Bottom-right
)
// Sum the values at the unique corner positions.
return corners.sumOf { (row, col) ->
matrix[row][col]
}
}
fun main() {
val matrix = listOf(
listOf(1, 2, 3, 4),
listOf(5, 6, 7, 8),
listOf(9, 10, 11, 12)
)
println("Sum of corners: ${sumCorners(matrix)}")
}
/*
run:
Sum of corners: 26
*/
