add: CountIslands

Author: Jer3myYu

java/Graphs/CountIslands.java

@@ -0,0 +1,39 @@
+public class CountIslands {
+    public int countIslands(int[][] matrix) {
+        if (matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) {
+            return 0;
+        }
+        int count = 0;
+        for (int r = 0; r < matrix.length; r++) {
+            for (int c = 0; c < matrix[0].length; c++) {
+                // If a land cell is found, perform DFS to explore the full 
+                // island, and include this island in our count.
+                if (matrix[r][c] == 1) {
+                    dfs(r, c, matrix);
+                    count++;
+                }
+            }
+        }
+        return count;
+    }
+
+    private void dfs(int r, int c, int[][] matrix) {
+        // Mark the current land cell as visited.
+        matrix[r][c] = -1;
+        // Define direction vectors for up, down, left, and right.
+        int[][] dirs = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
+        // Recursively call DFS on each neighboring land cell to continue 
+        // exploring this island.
+        for (int[] d : dirs) {
+            int nextR = r + d[0];
+            int nextC = c + d[1];
+            if (isWithinBounds(nextR, nextC, matrix) && matrix[nextR][nextC] == 1) {
+                dfs(nextR, nextC, matrix);
+            }
+        }
+    }
+
+    private boolean isWithinBounds(int r, int c, int[][] matrix) {
+        return 0 <= r && r < matrix.length && 0 <= c && c < matrix[0].length;
+    }
+}