SudokuSolver in kotlin

SudokuSolver.kt

class SudokuSolver(private val board: Array<IntArray>) {

    fun solve(): Boolean {
        val emptyCell = findEmptyCell() ?: return true

        for (number in 1..9) {
            if (isValid(emptyCell.first, emptyCell.second, number)) {
                board[emptyCell.first][emptyCell.second] = number

                if (solve()) {
                    return true
                }

                board[emptyCell.first][emptyCell.second] = 0 // Backtrack
            }
        }
        return false
    }

    private fun findEmptyCell(): Pair<Int, Int>? {
        for (i in board.indices) {
            for (j in board[i].indices) {
                if (board[i][j] == 0) {
                    return Pair(i, j)
                }
            }
        }
        return null
    }

    private fun isValid(row: Int, col: Int, number: Int): Boolean {
        return !isInRow(row, number) && !isInColumn(col, number) && !isInBox(row, col, number)
    }

    private fun isInRow(row: Int, number: Int): Boolean {
        return board[row].contains(number)
    }

    private fun isInColumn(col: Int, number: Int): Boolean {
        return board.any { it[col] == number }
    }

    private fun isInBox(row: Int, col: Int, number: Int): Boolean {
        val boxRowStart = row - row % 3
        val boxColStart = col - col % 3
        for (i in 0 until 3) {
            for (j in 0 until 3) {
                if (board[boxRowStart + i][boxColStart + j] == number) {
                    return true
                }
            }
        }
        return false
    }
}

// Example usage
fun main() {
    val board = arrayOf(
        intArrayOf(5, 3, 0, 0, 7, 0, 0, 0, 0),
        intArrayOf(6, 0, 0, 1, 9, 5, 0, 0, 0),
        intArrayOf(0, 9, 8, 0, 0, 0, 0, 6, 0),
        intArrayOf(8, 0, 0, 0, 6, 0, 0, 0, 3),
        intArrayOf(4, 0, 0, 8, 0, 3, 0, 0, 1),
        intArrayOf(7, 0, 0, 0, 2, 0, 0, 0, 6),
        intArrayOf(0, 6, 0, 0, 0, 0, 2, 8, 0),
        intArrayOf(0, 0, 0, 4, 1, 9, 0, 0, 5),
        intArrayOf(0, 0, 0, 0, 8, 0, 0, 7, 9)
    )

    val solver = SudokuSolver(board)
    if (solver.solve()) {
        println("Sudoku solved:")
        board.forEach { println(it.joinToString(" ")) }
    } else {
        println("No solution exists.")
    }
}