Update0037.解数独，添加C#

problems/0037.解数独.md

@@ -756,6 +756,59 @@ object Solution {
   }
 }
 ```
+### C#
+```csharp
+public class Solution
+{
+    public void SolveSudoku(char[][] board)
+    {
+        BackTracking(board);
+    }
+    public bool BackTracking(char[][] board)
+    {
+        for (int i = 0; i < board.Length; i++)
+        {
+            for (int j = 0; j < board[0].Length; j++)
+            {
+                if (board[i][j] != '.') continue;
+                for (char k = '1'; k <= '9'; k++)
+                {
+                    if (IsValid(board, i, j, k))
+                    {
+                        board[i][j] = k;
+                        if (BackTracking(board)) return true;
+                        board[i][j] = '.';
+                    }
+                }
+                return false;
+            }
+
+        }
+        return true;
+    }
+    public bool IsValid(char[][] board, int row, int col, char val)
+    {
+        for (int i = 0; i < 9; i++)
+        {
+            if (board[i][col] == val) return false;
+        }
+        for (int i = 0; i < 9; i++)
+        {
+            if (board[row][i] == val) return false;
+        }
+        int startRow = (row / 3) * 3;
+        int startCol = (col / 3) * 3;
+        for (int i = startRow; i < startRow + 3; i++)
+        {
+            for (int j = startCol; j < startCol + 3; j++)
+            {
+                if (board[i][j] == val) return false;
+            }
+        }
+        return true;
+    }
+}
+```
 
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