[20250818] BOJ / G4 / 미세먼지 안녕! / 이준희

JHLEE325/202508/18 BOJ G4 미세먼지 안녕!.md

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+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+    static int r, c, t;
+    static int[][] map;
+    static int up, down;
+    static final int[] dy = {-1, 1, 0, 0};
+    static final int[] dx = {0, 0, -1, 1};
+
+    public static void main(String[] args) throws Exception {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        StringTokenizer st = new StringTokenizer(br.readLine());
+        r = Integer.parseInt(st.nextToken());
+        c = Integer.parseInt(st.nextToken());
+        t = Integer.parseInt(st.nextToken());
+
+        map = new int[r][c];
+        up = down = -1;
+
+        for (int i = 0; i < r; i++) {
+            st = new StringTokenizer(br.readLine());
+            for (int j = 0; j < c; j++) {
+                map[i][j] = Integer.parseInt(st.nextToken());
+                if (map[i][j] == -1) {
+                    if (up == -1) up = i;
+                    else down = i;
+                }
+            }
+        }
+
+        while (t-- > 0) {
+            spread();
+            purify();
+        }
+
+        long sum = 0;
+        for (int i = 0; i < r; i++)
+            for (int j = 0; j < c; j++)
+                if (map[i][j] > 0) sum += map[i][j];
+
+        System.out.println(sum);
+    }
+
+    static void spread() {
+        int[][] add = new int[r][c];
+        for (int y = 0; y < r; y++) {
+            for (int x = 0; x < c; x++) {
+                if (map[y][x] <= 0) continue;
+                int amount = map[y][x] / 5;
+                if (amount == 0) continue;
+
+                int cnt = 0;
+                for (int d = 0; d < 4; d++) {
+                    int ny = y + dy[d], nx = x + dx[d];
+                    if (ny < 0 || nx < 0 || ny >= r || nx >= c) continue;
+                    if (map[ny][nx] == -1) continue;
+                    add[ny][nx] += amount;
+                    cnt++;
+                }
+                map[y][x] -= amount * cnt;
+            }
+        }
+        for (int i = 0; i < r; i++) {
+            for (int j = 0; j < c; j++) {
+                if (map[i][j] == -1) continue;
+                map[i][j] += add[i][j];
+            }
+        }
+    }
+
+    static void purify() {
+        for (int i = up - 1; i > 0; i--) map[i][0] = map[i - 1][0];
+        for (int j = 0; j < c - 1; j++) map[0][j] = map[0][j + 1];
+        for (int i = 0; i < up; i++) map[i][c - 1] = map[i + 1][c - 1];
+        for (int j = c - 1; j > 1; j--) map[up][j] = map[up][j - 1];
+        map[up][1] = 0;
+        map[up][0] = -1;
+
+        for (int i = down + 1; i < r - 1; i++) map[i][0] = map[i + 1][0];
+        for (int j = 0; j < c - 1; j++) map[r - 1][j] = map[r - 1][j + 1];
+        for (int i = r - 1; i > down; i--) map[i][c - 1] = map[i - 1][c - 1];
+        for (int j = c - 1; j > 1; j--) map[down][j] = map[down][j - 1];
+        map[down][1] = 0;
+        map[down][0] = -1;
+    }
+}
+
+```