Update Solution2.java

solution/0200-0299/0200.Number of Islands/Solution2.java

@@ -1,39 +1,32 @@
 class Solution {
-    private char[][] grid;
-    private int m;
-    private int n;
+  public int[] findOrder(int numCourses, int[][] prerequisites) {
+    List<Integer>[] graph = new List[numCourses];
+    int[] inDegrees = new int[numCourses];
 
-    public int numIslands(char[][] grid) {
-        m = grid.length;
-        n = grid[0].length;
-        this.grid = grid;
-        int ans = 0;
-        for (int i = 0; i < m; ++i) {
-            for (int j = 0; j < n; ++j) {
-                if (grid[i][j] == '1') {
-                    bfs(i, j);
-                    ++ans;
-                }
-            }
-        }
-        return ans;
+    for (int i = 0; i < numCourses; ++i)
+      graph[i] = new ArrayList<>();
+
+    for (int[] prerequisite : prerequisites) {
+      graph[prerequisite[1]].add(prerequisite[0]);
+      ++inDegrees[prerequisite[0]];
     }
 
-    private void bfs(int i, int j) {
-        grid[i][j] = '0';
-        Deque<int[]> q = new ArrayDeque<>();
-        q.offer(new int[] {i, j});
-        int[] dirs = {-1, 0, 1, 0, -1};
-        while (!q.isEmpty()) {
-            int[] p = q.poll();
-            for (int k = 0; k < 4; ++k) {
-                int x = p[0] + dirs[k];
-                int y = p[1] + dirs[k + 1];
-                if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] == '1') {
-                    q.offer(new int[] {x, y});
-                    grid[x][y] = '0';
-                }
-            }
-        }
+    int[] ans = new int[numCourses];
+    int index = 0;
+
+    Queue<Integer> q = new LinkedList<>();
+    for (int i = 0; i < numCourses; ++i)
+      if (inDegrees[i] == 0)
+        q.offer(i);
+
+    while (!q.isEmpty()) {
+      int u = q.poll();
+      ans[index++] = u;
+      for (int v : graph[u])
+        if (--inDegrees[v] == 0)
+          q.offer(v);
     }
-}
+
+    return index == numCourses ? ans : new int[] {};
+  }
+}