Rotate an Array §

Rotating an array by 90${}^{\circ }$ is a common question which has a bit of a trick to it.

The key is you can achieve this by transposing, then reflecting the matrix. Transposing means to swap each entry about the diagonal, and reflecting means reversing the array from left to right.

fun rotate(matrix: Array<IntArray>): Unit {
	transpose(matrix)
	reflect(matrix)
}
 
private fun transpose(matrix: Array<IntArray>): Unit {
	val length = matrix.size
	
	for (row in 0 until length) {
		// Note that we start iterating through columns at position == row
		// This is because we are working diagonally
		for (column in row until length) {
			val temp = matrix[row][column]
			matrix[row][column] = matrix[column][row]
			matrix[column][row] = temp
		}
	}
}
 
private fun reflect(matrix: Array<IntArray>): Unit {
	val length = matrix.size
 
	for (row in 0 until length) {
		// Note that we reflect until length / 2
		// No reason to keep going after halfway
		for (column in 0 until length / 2) {
			val temp = matrix[row][column]
			matrix[row][column] = matrix[row][length - column - 1]
			matrix[row][length - column - 1] = temp
		}
	}
}