feat: number-of-islands

Author: minji-go

number-of-islands/minji-go.java

@@ -0,0 +1,57 @@
+/**
+ * <a href="https://leetcode.com/problems/number-of-islands/">week07-3. number-of-islands</a>
+ * <li>Description: return the number of islands is surrounded by water("0")        </li>
+ * <li>Topics: Array, Depth-First Search, Breadth-First Search, Union Find, Matrix  </li>
+ * <li>Time Complexity: O(M×N), Runtime 5ms     </li>
+ * <li>Space Complexity: O(M×N), Memory 52.11MB </li>
+ */
+class Solution {
+    private char[][] grid;
+    private int m;
+    private int n;
+    private boolean[][] visit;
+    private int[][] directions = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
+
+    public int numIslands(char[][] grid) {
+        this.grid = grid;
+        this.m = grid.length;
+        this.n = grid[0].length;
+        this.visit = new boolean[m][n];
+
+        int num = 0;
+        for (int r = 0; r < m; r++) {
+            for (int c = 0; c < n; c++) {
+                if (grid[r][c] == '1' && !visit[r][c]) {
+                    findIsland(r, c);
+                    num++;
+                }
+            }
+        }
+        return num;
+    }
+
+    public void findIsland(int r, int c) {
+        Queue<int[]> queue = new LinkedList<>();
+        visit[r][c] = true;
+        queue.offer(new int[]{r, c});
+
+        while (!queue.isEmpty()) {
+            int[] cur = queue.poll();
+            int cr = cur[0];
+            int cc = cur[1];
+
+            for (int[] dir : directions) {
+                int nr = cr + dir[0];
+                int nc = cc + dir[1];
+                if (nr < 0 || nr > m - 1 || nc < 0 || nc > n - 1) {
+                    continue;
+                }
+                if (grid[nr][nc] == '1' && !visit[nr][nc]) {
+                    visit[nr][nc] = true;
+                    queue.offer(new int[]{nr, nc});
+                }
+            }
+        }
+    }
+
+}