Sudoku.swift

import Foundation

struct SudokuBoard {
    var board: [Int]
    
    subscript(_ x: Int, _ y: Int) -> Int {
        get {
            return board[y * 9 + x]
        }
        set {
            board[y * 9 + x] = newValue
        }
    }
    
    init(board: [[Int]]) {
        self.board = [Int](repeating: 0, count: 9 * 9)
        for y in 0..<9 {
            for x in 0..<9 {
                self[x, y] = board[y][x]
            }
        }
    }
    
    func isValid(number: Int, at point: (x: Int, y: Int)) -> Bool {
        for i in 0..<9 {
            if self[point.x, i] == number { return false }
            if self[i, point.y] == number { return false }
        }
        for x in 0..<3 {
            for y in 0..<3 {
                if self[(point.x / 3) * 3 + x, (point.y / 3) * 3 + y] == number { return false }
            }
        }
        return true
    }
    
    func children() -> [SudokuBoard]? {
        guard let idx = board.firstIndex(of: 0) else {
            return nil
        }
        var children: [SudokuBoard] = []
        for i in 1...9 {
            if isValid(number: i, at: (idx % 9, idx / 9)) {
                var newBoard = self
                newBoard.board[idx] = i
                children.append(newBoard)
            }
        }
        return children
    }
    
    func solve() -> SudokuBoard? {
        guard let children = children() else {
            return self
        }
        
        if children.isEmpty {
            return nil
        }
        
        for child in children {
            if let solution = child.solve() {
                return solution
            }
        }
        
        return nil
    }
}

let board = SudokuBoard(board: [
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0],
])

if let solution = board.solve() {
    for y in 0..<9 {
        for x in 0..<9 {
            print(solution[x, y], terminator: "|")
        }
        print("\n\([String](repeating: "-", count: 17).joined())")
    }
    print("\n\n")
}