Added one problem- Number of Islands (#32)

Author: baig62

Java/NumberOfIslands.java

@@ -0,0 +1,76 @@
+/**
+ * Given a 2d grid map of '1's (land) and '0's (water), count the number of
+ * islands. An island is surrounded by water and is formed by connecting
+ * adjacent lands horizontally or vertically. You may assume all four edges of
+ * the grid are all surrounded by water.
+ * 
+ * Problem: https://leetcode.com/problems/number-of-islands/submissions/
+ * 
+ * Submission link: https://leetcode.com/submissions/detail/385691397/
+ * 
+ * Runtime: 1 ms, faster than 99.94% of Java online submissions
+ * 
+ * Memory Usage: 42.3 MB, less than 52.50% of Java online submissions
+ * 
+ * @author Mirzafazulur Rahamanbaig
+ *
+ */
+class NumberOfIslands {
+
+	boolean[][] visited;
+	int r, c;
+	int islands;
+
+	/**
+	 * 
+	 * @param grid
+	 * @return
+	 */
+	public int numIslands(char[][] grid) {
+
+		if (grid == null || grid.length == 0) {
+
+			return 0;
+		}
+
+		r = grid.length;
+		c = grid[0].length;
+		islands = 0;
+		visited = new boolean[r][c];
+
+		for (int i = 0; i < r; i++) {
+
+			for (int j = 0; j < c; j++) {
+
+				if (grid[i][j] == '1' && !visited[i][j]) {
+
+					_numIslands(grid, i, j);
+					islands++;
+				}
+			}
+		}
+
+		return islands;
+	}
+
+	/**
+	 * 
+	 * @param grid
+	 * @param m
+	 * @param n
+	 */
+	private void _numIslands(char[][] grid, int m, int n) {
+
+		if (m < 0 || m >= r || n < 0 || n >= c || visited[m][n] || grid[m][n] == '0') {
+
+			return;
+		}
+
+		visited[m][n] = true;
+
+		_numIslands(grid, m + 1, n);
+		_numIslands(grid, m, n + 1);
+		_numIslands(grid, m - 1, n);
+		_numIslands(grid, m, n - 1);
+	}
+}

README.md

@@ -204,6 +204,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | #   | Title                                                             | Solution                            | Time   | Space  | Difficulty | Tag  | Note |
 | --- | ----------------------------------------------------------------- | ----------------------------------- | ------ | ------ | ---------- | ---- | ---- |
 | 1284 | [Minimum Number of Flips to Convert Binary Matrix to Zero Matrix](https://leetcode.com/problems/minimum-number-of-flips-to-convert-binary-matrix-to-zero-matrix/) | [C++](./C++/Minimum-Number-of-Flips-to-Convert-Binary-Matrix-to-Zero-Matrix.cpp) | _O(m * n * 2 ^ (m * n))_ | _O(2 ^ (m * n))_ | Hard     | BFS |      |
+| 200 | [Number of Islands](https://leetcode.com/problems/number-of-islands/) | [Java](./Java/NumberOfIslands.java) | O(R * C) | O(R * C) | Medium | BFS |
 
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