solves number of islands in java

Author: anishLearnsToCode

README.md

@@ -159,7 +159,7 @@
 | 191  | [Number of One Bits](https://leetcode.com/problems/number-of-1-bits)                                                                                      |                                     [![Java](assets/java.png)](src/NumberOf1Bit.java) [![Python](assets/python.png)](python/number_of_1_bits.py)                                     |                                                                                                                                   |
 | 198  | [House Robber](https://leetcode.com/problems/house-robber)                                                                                                |                                       [![Java](assets/java.png)](src/HouseRobber.java) [![Python](assets/python.png)](python/house_robber.py)                                        |                                                                                                                                   |
 | 199  | [Binary Tree Right Side View](https://leetcode.com/problems/binary-tree-right-side-view)                                                                  |                                                             [![Java](assets/java.png)](src/BinaryTreeRightSideView.java)                                                             |                                                                                                                                   |
-| 200  | [Number of Islands](https://leetcode.com/problems/number-of-islands)                                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
+| 200  | [Number of Islands](https://leetcode.com/problems/number-of-islands)                                                                                      |                                                                 [![Java](assets/java.png)](src/NumberOfIslands.java)                                                                 |                                                                                                                                   |
 | 201  | [Bitwise AND of Numbers Range](https://leetcode.com/problems/bitwise-and-of-numbers-range)                                                                |                                                                                                                                                                                      |                                                                                                                                   |
 | 202  | [Happy Number](https://leetcode.com/problems/happy-number)                                                                                                |                                       [![Java](assets/java.png)](src/HappyNumber.java) [![Python](assets/python.png)](python/happy_number.py)                                        |                                                                                                                                   |
 | 203  | [Remove Linked List Elements](https://leetcode.com/problems/remove-linked-list-elements)                                                                  |                         [![Java](assets/java.png)](src/RemoveLinkedListElements.java) [![Python](assets/python.png)](python/remove_linked_list_elements.py)                          |                                                                                                                                   |

src/NumberOfIslands.java

@@ -0,0 +1,52 @@
+// https://leetcode.com/problems/number-of-islands
+// T: O(m * n)
+// T: O(m * n)
+
+public class NumberOfIslands {
+    private static final char VISITED = '0';
+    private static final char ISLAND = '1';
+
+    public int numIslands(char[][] grid) {
+        final int rows = grid.length, columns = grid[0].length;
+        int islands = 0;
+
+        for (int row = 0 ; row < rows ; row++) {
+            for (int column = 0 ; column < columns ; column++) {
+                if (isIsland(grid[row][column])) {
+                    islands++;
+                    markIslandAsSeen(grid, row, column);
+                }
+            }
+        }
+
+        return islands;
+    }
+
+    private boolean isIsland(char c) {
+        return c == ISLAND;
+    }
+
+    private void markIslandAsSeen(char[][] grid, int row, int column) {
+        if (!isValidPosition(grid, row, column) || !isIsland(grid[row][column])) return;
+        markVisited(grid, row, column);
+        markIslandAsSeen(grid, row - 1, column);
+        markIslandAsSeen(grid, row, column + 1);
+        markIslandAsSeen(grid, row + 1, column);
+        markIslandAsSeen(grid, row, column - 1);
+    }
+
+    private boolean isValidPosition(char[][] grid, int row, int column) {
+        return row >= 0
+                && row < grid.length
+                && column >= 0
+                && column < grid[0].length;
+    }
+
+    private boolean alreadyVisited(char c) {
+        return c == VISITED;
+    }
+
+    private void markVisited(char[][] grid, int row, int column) {
+        grid[row][column] = VISITED;
+    }
+}