https://leetcode.com/problems/surrounded-regions/

Author: harshitgindra

LeetCodeSolutions/Graph/SurroundedRegions.cs

@@ -0,0 +1,88 @@
+namespace LeetCodeSolutions.Graph;
+
+public class SurroundedRegions
+{
+    public void Solve(char[][] board)
+    {
+        // Handle edge cases
+        if (board == null || board.Length == 0) return;
+
+        int rows = board.Length;
+        int cols = board[0].Length;
+
+        // Step 1: Mark all 'O's connected to borders as safe ('T')
+        // Check first and last column
+        for (int row = 0; row < rows; row++)
+        {
+            MarkSafeRegion(board, row, 0);
+            MarkSafeRegion(board, row, cols - 1);
+        }
+
+        // Check first and last row
+        for (int col = 0; col < cols; col++)
+        {
+            MarkSafeRegion(board, 0, col);
+            MarkSafeRegion(board, rows - 1, col);
+        }
+
+        // Step 2: Convert all remaining 'O's to 'X' and restore 'T's to 'O'
+        for (int row = 0; row < rows; row++)
+        {
+            for (int col = 0; col < cols; col++)
+            {
+                if (board[row][col] == 'O')
+                {
+                    // This 'O' is surrounded, so capture it
+                    board[row][col] = 'X';
+                }
+                else if (board[row][col] == 'T')
+                {
+                    // This was a safe 'O', restore it
+                    board[row][col] = 'O';
+                }
+            }
+        }
+    }
+
+    private void MarkSafeRegion(char[][] board, int row, int col)
+    {
+        int rows = board.Length;
+        int cols = board[0].Length;
+
+        // Base case: out of bounds or not an 'O'
+        if (row < 0 || row >= rows || col < 0 || col >= cols || board[row][col] != 'O')
+        {
+            return;
+        }
+
+        // Mark this cell as safe
+        board[row][col] = 'T';
+
+        // Check all four adjacent cells
+        MarkSafeRegion(board, row + 1, col); // Down
+        MarkSafeRegion(board, row - 1, col); // Up
+        MarkSafeRegion(board, row, col + 1); // Right
+        MarkSafeRegion(board, row, col - 1); // Left
+    }
+
+
+    [Test(Description = "https://leetcode.com/problems/surrounded-regions/")]
+    [Category("Medium")]
+    [Category("LeetCode")]
+    [Category("Surrounded Regions")]
+    [TestCaseSource(nameof(Input))]
+    [Category("Graph")]
+    [Category("TopInterview")]
+    public void Test1((char[][] Output, char[][] Input) item)
+    {
+        this.Solve(item.Input);
+        Assert.That(item.Input, Is.EqualTo(item.Output));
+    }
+
+    public static IEnumerable<(char[][] Output, char[][] Input)> Input =>
+        new List<(char[][] Output, char[][] Input)>()
+        {
+            ([['X', 'X', 'X', 'X'], ['X', 'X', 'X', 'X'], ['X', 'X', 'X', 'X'], ['X', 'O', 'X', 'X']],
+                [['X', 'X', 'X', 'X'], ['X', 'O', 'O', 'X'], ['X', 'X', 'O', 'X'], ['X', 'O', 'X', 'X']])
+        };
+}

README.md

@@ -143,8 +143,8 @@ The Top Interview 150 collection on Leetcode is a curated set of 150 interview q
 | 87                                           | Kth Smallest Element in a BST                                                                             | Medium     |        |
 | 98                                           | Validate Binary Search Tree                                                                               | Medium     |   ✅     |
 | <br>        Graph General<br>                |                                                                                                           |            |        |
-| 89                                           | Number of Islands                                                                                         | Medium     |   ✅     |
-| 90                                           | Surrounded Regions                                                                                        | Medium     |        |
+| 200                                           | Number of Islands                                                                                         | Medium     |   ✅     |
+| 130                                           | Surrounded Regions                                                                                        | Medium     |    ✅    |
 | 91                                           | Clone Graph                                                                                               | Medium     |        |
 | 92                                           | Evaluate Division                                                                                         | Medium     |        |
 | 93                                           | Course Schedule                                                                                           | Medium     |        |