优化kamacoder 101孤岛的总面积 Java BFS版本注释及逻辑

problems/kamacoder/0101.孤岛的总面积.md

@@ -189,69 +189,70 @@ int main() {
 import java.util.*;
 
 public class Main {
-    private static int count = 0;
-    private static final int[][] dir = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}}; // 四个方向
-
-    private static void bfs(int[][] grid, int x, int y) {
-        Queue<int[]> que = new LinkedList<>();
-        que.add(new int[]{x, y});
-        grid[x][y] = 0; // 只要加入队列，立刻标记
-        count++;
-        while (!que.isEmpty()) {
-            int[] cur = que.poll();
-            int curx = cur[0];
-            int cury = cur[1];
+    static int area = 0;
+    static int[][] directions = { { 1, 0 }, { 0, 1 }, { -1, 0 }, { 0, -1 } };
+
+    // BFS：从(x,y)出发，将所有与之相连的陆地标记为0（消除）
+    public static void bfs(int[][] graph, int x, int y) {
+        Queue<int[]> queue = new LinkedList<>();
+        queue.add(new int[] { x, y });
+        graph[x][y] = 0; // 入队立刻标记，避免重复访问
+        while (!queue.isEmpty()) {
+            int[] cur = queue.poll();
+            int curX = cur[0];
+            int curY = cur[1];
             for (int i = 0; i < 4; i++) {
-                int nextx = curx + dir[i][0];
-                int nexty = cury + dir[i][1];
-                if (nextx < 0 || nextx >= grid.length || nexty < 0 || nexty >= grid[0].length) continue; // 越界了，直接跳过
-                if (grid[nextx][nexty] == 1) {
-                    que.add(new int[]{nextx, nexty});
-                    count++;
-                    grid[nextx][nexty] = 0; // 只要加入队列立刻标记
+                int nextX = curX + directions[i][0];
+                int nextY = curY + directions[i][1];
+                // 越界跳过
+                if (nextX < 0 || nextX >= graph.length || nextY < 0 || nextY >= graph[0].length) {
+                    continue;
+                }
+                if (graph[nextX][nextY] == 1) {
+                    queue.add(new int[] { nextX, nextY });
+                    graph[nextX][nextY] = 0; // 入队立刻标记
                 }
             }
         }
     }
 
     public static void main(String[] args) {
-        Scanner scanner = new Scanner(System.in);
-        int n = scanner.nextInt();
-        int m = scanner.nextInt();
-        int[][] grid = new int[n][m];
-
-        // 读取网格
+        Scanner sc = new Scanner(System.in);
+        int n = sc.nextInt();
+        int m = sc.nextInt();
+        int[][] graph = new int[n][m];
+
         for (int i = 0; i < n; i++) {
             for (int j = 0; j < m; j++) {
-                grid[i][j] = scanner.nextInt();
+                graph[i][j] = sc.nextInt();
             }
         }
-
-        // 从左侧边，和右侧边向中间遍历
+
+        // 第一阶段：从四条边出发，消除所有与边缘相连的陆地
+        // 这些陆地不是孤岛，直接标记为0
         for (int i = 0; i < n; i++) {
-            if (grid[i][0] == 1) bfs(grid, i, 0);
-            if (grid[i][m - 1] == 1) bfs(grid, i, m - 1);
+            if (graph[i][0] == 1) bfs(graph, i, 0);         // 左边
+            if (graph[i][m - 1] == 1) bfs(graph, i, m - 1); // 右边
         }
-
-        // 从上边和下边向中间遍历
-        for (int j = 0; j < m; j++) {
-            if (grid[0][j] == 1) bfs(grid, 0, j);
-            if (grid[n - 1][j] == 1) bfs(grid, n - 1, j);
+        for (int i = 0; i < m; i++) {
+            if (graph[0][i] == 1) bfs(graph, 0, i);         // 上边
+            if (graph[n - 1][i] == 1) bfs(graph, n - 1, i); // 下边
         }
-        
-        count = 0;
+
+        // 第二阶段：统计剩余陆地面积，即孤岛总面积
         for (int i = 0; i < n; i++) {
             for (int j = 0; j < m; j++) {
-                if (grid[i][j] == 1) bfs(grid, i, j);
+                if (graph[i][j] == 1) {
+                    area++;
+                }
             }
         }
 
-        System.out.println(count);
+        System.out.println(area);
     }
 }
 
 
-
 ```