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| 1 | +# 37. Sudoku Solver |
| 2 | + |
| 3 | +### 2020年07月29日 |
| 4 | + |
| 5 | +Write a program to solve a Sudoku puzzle by filling the empty cells. |
| 6 | + |
| 7 | +A sudoku solution must satisfy **all of the following rules**: |
| 8 | + |
| 9 | +1. Each of the digits `1-9` must occur exactly once in each row. |
| 10 | +2. Each of the digits `1-9` must occur exactly once in each column. |
| 11 | +3. Each of the the digits `1-9` must occur exactly once in each of the 9 `3x3` sub-boxes of the grid. |
| 12 | + |
| 13 | +Empty cells are indicated by the character `'.'`. |
| 14 | + |
| 15 | + |
| 16 | +A sudoku puzzle... |
| 17 | + |
| 18 | + |
| 19 | +...and its solution numbers marked in red. |
| 20 | + |
| 21 | +**Note:** |
| 22 | + |
| 23 | +- The given board contain only digits `1-9` and the character `'.'`. |
| 24 | +- You may assume that the given Sudoku puzzle will have a single unique solution. |
| 25 | +- The given board size is always `9x9`. |
| 26 | + |
| 27 | + |
| 28 | +# Solution |
| 29 | + |
| 30 | +```swift |
| 31 | + |
| 32 | +class Solution { |
| 33 | + static let num: [Character] = ["1", "2", "3", "4", "5", "6", "7", "8", "9"] |
| 34 | + func solveSudoku(_ board: inout [[Character]]) { |
| 35 | + var clo = [Set<Character>].init(repeating: Set<Character>(), count: 9) |
| 36 | + var row = [Set<Character>].init(repeating: Set<Character>(), count: 9) |
| 37 | + var block = [Set<Character>].init(repeating: Set<Character>(), count: 9) |
| 38 | + for y in 0..<9 { |
| 39 | + for x in 0..<9 { |
| 40 | + let c = board[y][x] |
| 41 | + clo[y].insert(c) |
| 42 | + row[x].insert(c) |
| 43 | + block[x/3 + (y/3) * 3].insert(c) |
| 44 | + } |
| 45 | + } |
| 46 | + _ = fillGrid(index: 0, board: &board, clo: &clo, row: &row, block: &block) |
| 47 | + } |
| 48 | + |
| 49 | + |
| 50 | + func fillGrid(index: Int, |
| 51 | + board: inout [[Character]], |
| 52 | + clo: inout [Set<Character>], |
| 53 | + row: inout [Set<Character>], |
| 54 | + block: inout [Set<Character>]) -> Bool { |
| 55 | + guard index < 81 else { |
| 56 | + return true |
| 57 | + } |
| 58 | + let x = index % 9 |
| 59 | + let y = index / 9 |
| 60 | + if board[y][x] == "." { |
| 61 | + for c in Solution.num { |
| 62 | + if !clo[y].contains(c), !row[x].contains(c), !block[x/3 + (y/3) * 3].contains(c) { |
| 63 | + board[y][x] = c |
| 64 | + clo[y].insert(c) |
| 65 | + row[x].insert(c) |
| 66 | + block[x/3 + (y/3) * 3].insert(c) |
| 67 | + if fillGrid(index: index + 1, board: &board, clo: &clo, row: &row, block: &block) { |
| 68 | + return true |
| 69 | + } else { |
| 70 | + clo[y].remove(c) |
| 71 | + row[x].remove(c) |
| 72 | + block[x/3 + (y/3) * 3].remove(c) |
| 73 | + board[y][x] = "." |
| 74 | + } |
| 75 | + } |
| 76 | + } |
| 77 | + return false |
| 78 | + } else { |
| 79 | + return fillGrid(index: index + 1, board: &board, clo: &clo, row: &row, block: &block) |
| 80 | + } |
| 81 | + } |
| 82 | +} |
| 83 | + |
| 84 | +``` |
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