0037.解数独：优化排版，补充Swift版本

problems/0037.解数独.md

@@ -61,7 +61,7 @@
 
 代码如下：
 
-```
+```cpp
 bool backtracking(vector<vector<char>>& board)
 ```
 
@@ -504,5 +504,54 @@ void solveSudoku(char** board, int boardSize, int* boardColSize) {
 }
 ```
 
+### Swift
+
+```swift
+func solveSudoku(_ board: inout [[Character]]) {
+    // 判断对应格子的值是否合法
+    func isValid(row: Int, col: Int, val: Character) -> Bool {
+        // 行中是否重复
+        for i in 0 ..< 9 {
+            if board[row][i] == val { return false }
+        }
+
+        // 列中是否重复
+        for j in 0 ..< 9 {
+            if board[j][col] == val { return false }
+        }
+
+        // 9方格内是否重复
+        let startRow = row / 3 * 3
+        let startCol = col / 3 * 3
+        for i in startRow ..< startRow + 3 {
+            for j in startCol ..< startCol + 3 {
+                if board[i][j] == val { return false }
+            }
+        }
+        return true
+    }
+
+    @discardableResult
+    func backtracking() -> Bool {
+        for i in 0 ..< board.count { // i：行坐标
+            for j in 0 ..< board[0].count { // j：列坐标
+                guard board[i][j] == "." else { continue } // 跳过已填写格子
+                // 填写格子
+                for val in 1 ... 9 {
+                    let charVal = Character("\(val)")
+                    guard isValid(row: i, col: j, val: charVal) else { continue } // 跳过不合法的
+                    board[i][j] = charVal // 填写
+                    if backtracking() { return true }
+                    board[i][j] = "." // 回溯：擦除
+                }
+                return false // 遍历完数字都不行
+            }
+        }
+        return true // 没有不合法的，填写正确
+    }
+    backtracking()
+}
+```
+
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