Add count the islands problem.

src/hacker-rank/CountTheIslands.cs

+namespace CodingInterview.HackerRank
+{
+	/// <summary>
+	/// Given a boolean matrix, find the number of islands.
+	///
+	/// What is an island?
+	///
+	/// A group of connected 1s forms an island.
+	/// For example, the below matrix contains 5 islands:
+	///
+	/// {1, 1, 0, 0, 0},
+	/// {0, 1, 0, 0, 1},
+	/// {1, 0, 0, 1, 1},
+	/// {0, 0, 0, 0, 0},
+	/// {1, 0, 1, 0, 1}
+	/// </summary>
+	public class CountTheIslands
+	{
+		static int[,] map;
+
+		/// <summary>
+		/// Returns the number of islands
+		/// </summary>
+		/// <param name="map">the map matrix</param>
+		/// <returns>the number of islands</returns>
+		public int Solve(int[,] map)
+		{
+			CountTheIslands.map = map;
+
+			var islands = 0;
+			for (var i = 0; i < map.GetLength(0); i++)
+			{
+				for (var j = 0; j < map.GetLength(1); j++)
+				{
+					if (DFS(i, j))
+					{
+						islands++;
+					}
+				}
+			}
+
+			return islands;
+		}
+
+		static bool DFS(int i, int j)
+		{
+			if (i < 0 || i == map.GetLength(0) || j < 0 || j == map.GetLength(1) || map[i, j] == 0 || map[i, j] == 2)
+			{
+				return false;
+			}
+
+			map[i, j] = 2;
+
+			DFS(i, j + 1); // right
+			DFS(i + 1, j + 1); // right down
+			DFS(i + 1, j); // down
+			DFS(i + 1, j - 1); // left down
+			DFS(i, j - 1); // left
+			DFS(i - 1, j - 1); // left up
+			DFS(i - 1, j); // up
+			DFS(i - 1, j + 1); // right up
+
+			return true;
+		}
+	}
+}

test/hacker-rank/CountTheIslands.cs

+using Xunit;
+
+namespace CodingInterview.Tests.HackerRank
+{
+	public class CountTheIslands
+	{
+		[Fact]
+		public void TestCases()
+		{
+			// From Hacker Rank
+			Assert.Equal(
+				5,
+				new CodingInterview.HackerRank.CountTheIslands().Solve(
+					new int[,] {
+						{1, 1, 0, 0, 0},
+						{0, 1, 0, 0, 1},
+						{1, 0, 0, 1, 1},
+						{0, 0, 0, 0, 0},
+						{1, 0, 1, 0, 1}
+					}
+				)
+			);
+		}
+	}
+}