Update0051.N皇后，添加C#

problems/0051.N皇后.md

@@ -865,6 +865,60 @@ object Solution {
   }
 }
 ```
+### C#
+```csharp
+public class Solution
+{
+    public List<IList<string>> res = new();
+    public IList<IList<string>> SolveNQueens(int n)
+    {
+        char[][] chessBoard = new char[n][];
+        for (int i = 0; i < n; i++)
+        {
+            chessBoard[i] = new char[n];
+            for (int j = 0; j < n; j++)
+            {
+                chessBoard[i][j] = '.';
+            }
+        }
+        BackTracking(n, 0, chessBoard);
+        return res;
+    }
+    public void BackTracking(int n, int row, char[][] chessBoard)
+    {
+        if (row == n)
+        {
+            res.Add(chessBoard.Select(x => new string(x)).ToList());
+            return;
+        }
+        for (int col = 0; col < n; col++)
+        {
+            if (IsValid(row, col, chessBoard, n))
+            {
+                chessBoard[row][col] = 'Q';
+                BackTracking(n, row + 1, chessBoard);
+                chessBoard[row][col] = '.';
+            }
+        }
+    }
+    public bool IsValid(int row, int col, char[][] chessBoard, int n)
+    {
+        for (int i = 0; i < row; i++)
+        {
+            if (chessBoard[i][col] == 'Q') return false;
+        }
+        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--)
+        {
+            if (chessBoard[i][j] == 'Q') return false;
+        }
+        for (int i = row - 1, j = col + 1; i >= 0 && j < n; i--, j++)
+        {
+            if (chessBoard[i][j] == 'Q') return false;
+        }
+        return true;
+    }
+}
+```
 
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