agc033

A题意：给你个黑白矩阵，每次黑的周围一圈会变黑，求多少次之后全黑。n <= 1000

解：BFS即可。

 1 #include <bits/stdc++.h>
 2  
 3 const int N = 1010;
 4 const int dx[] = {1, 0, -1, 0};
 5 const int dy[] = {0, 1, 0, -1};
 6  
 7 struct Node {
 8     int x, y;
 9     Node(int X = 0, int Y = 0) {
10         x = X;
11         y = Y;
12     }
13 };
14  
15 int vis[N][N], d[N][N];
16 char str[N];
17 std::queue<Node> Q;
18  
19 int main() {
20  
21     int n, m, ans = 0;
22     scanf("%d%d", &n, &m);
23     for(int i = 1; i <= n; i++) {
24         scanf("%s", str + 1);
25         for(int j = 1; j <= m; j++) {
26             if(str[j] == '#') {
27                 Q.push(Node(i, j));
28                 vis[i][j] = 1;
29                 d[i][j] = 0;
30             }
31         }
32     }
33  
34     while(Q.size()) {
35         int x = Q.front().x, y = Q.front().y;
36         Q.pop();
37         for(int i = 0; i < 4; i++) {
38             int tx = x + dx[i], ty = y + dy[i];
39             if(tx < 1 || ty < 1 || tx > n || ty > m || vis[tx][ty]) {
40                 continue;
41             }
42             vis[tx][ty] = 1;
43             d[tx][ty] = d[x][y] + 1;
44             Q.push(Node(tx, ty));
45             ans = std::max(ans, d[tx][ty]);
46         }
47     }
48     printf("%d\n", ans);
49     return 0;
50 }

AC代码