Create 37. Sudoku Solver.md

Author: TBXark

37. Sudoku Solver.md

@@ -0,0 +1,84 @@
+# 37. Sudoku Solver
+
+### 2020-07-29
+
+Write a program to solve a Sudoku puzzle by filling the empty cells.
+
+A sudoku solution must satisfy **all of the following rules**:
+
+1. Each of the digits `1-9` must occur exactly once in each row.
+2. Each of the digits `1-9` must occur exactly once in each column.
+3. Each of the the digits `1-9` must occur exactly once in each of the 9 `3x3` sub-boxes of the grid.
+
+Empty cells are indicated by the character `'.'`.
+
+![img](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)
+A sudoku puzzle...
+
+![img](https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sudoku-by-L2G-20050714_solution.svg/250px-Sudoku-by-L2G-20050714_solution.svg.png)
+...and its solution numbers marked in red.
+
+**Note:**
+
+- The given board contain only digits `1-9` and the character `'.'`.
+- You may assume that the given Sudoku puzzle will have a single unique solution.
+- The given board size is always `9x9`.
+
+
+# Solution
+
+```swift
+
+class Solution {
+    static let num: [Character] = ["1", "2", "3", "4", "5", "6", "7", "8", "9"]
+    func solveSudoku(_ board: inout [[Character]]) {
+        var clo = [Set<Character>].init(repeating: Set<Character>(), count: 9)
+        var row = [Set<Character>].init(repeating: Set<Character>(), count: 9)
+        var block = [Set<Character>].init(repeating: Set<Character>(), count: 9)
+        for y in 0..<9 {
+            for x in 0..<9 {
+                let c = board[y][x]
+                clo[y].insert(c)
+                row[x].insert(c)
+                block[x/3 + (y/3) * 3].insert(c)
+            }
+        }
+        _ = fillGrid(index: 0, board: &board, clo: &clo, row: &row, block: &block)
+    }
+
+
+    func fillGrid(index: Int,
+                  board: inout [[Character]],
+                  clo: inout [Set<Character>],
+                  row: inout [Set<Character>],
+                  block: inout [Set<Character>]) -> Bool {
+        guard index < 81 else {
+            return true
+        }
+        let x = index % 9
+        let y = index / 9
+        if board[y][x] == "." {
+            for c in Solution.num {
+                if !clo[y].contains(c), !row[x].contains(c), !block[x/3 + (y/3) * 3].contains(c) {
+                    board[y][x] = c
+                    clo[y].insert(c)
+                    row[x].insert(c)
+                    block[x/3 + (y/3) * 3].insert(c)
+                    if fillGrid(index: index + 1, board: &board, clo: &clo, row: &row, block: &block) {
+                        return true
+                    } else {
+                        clo[y].remove(c)
+                        row[x].remove(c)
+                        block[x/3 + (y/3) * 3].remove(c)
+                        board[y][x] = "."
+                    }
+                }
+            }
+            return false
+        } else {
+            return fillGrid(index: index + 1, board: &board, clo: &clo, row: &row, block: &block)
+        }
+    }
+}
+
+```