UVA 11624 Fire! (bfs)

算法指南白书

分别求一次人和火到达各个点的最短时间

 #include<cstdio>
 #include<cstring>
 #include<queue>
 #include<algorithm>
 using namespace std;
 
 const int INF = ;
 const int maxr =  + ;
 const int maxc =  + ;
 int R, C;
 char maze[maxr][maxc];
 
 struct Cell {
     int r, c;
     Cell(int r, int c):r(r),c(c) {}
 };
 
 const int dr[] = {-,,,};
 const int dc[] = {,,-,};
 int d[maxr][maxc][], vis[maxr][maxc][];//0表示人，1表示火走
 
 queue<Cell> Q;
 void bfs(int kind) {
     while(!Q.empty()) {
         Cell cell = Q.front();
         Q.pop();
         int r = cell.r, c = cell.c;
         for(int dir = ; dir < ; dir++) {
             int nr = r + dr[dir], nc = c + dc[dir];
             if(nr >=  && nr < R && nc >=  && nc < C && maze[nr][nc] == '.' && !vis[nr][nc][kind]) {
                 Q.push(Cell(nr, nc));
                 vis[nr][nc][kind] = ;
                 d[nr][nc][kind] = d[r][c][kind] + ;
             }
         }
     }
 }
 
 int ans;
 void check(int r, int c) {
     if(maze[r][c] != '.' || !vis[r][c][]) return; // 必须是Joe可达的边界格子
     if(!vis[r][c][] || d[r][c][] < d[r][c][]) ans = min(ans, d[r][c][] + ); // Joe必须先于火到达
 }
 
 int main() {
     int T;
     scanf("%d", &T);
     while(T--) {
         scanf("%d%d", &R, &C);
         int jr, jc;
         vector<Cell> fires;
         for(int i = ; i < R; i++) {
             scanf("%s", maze[i]);
             for(int j = ; j < C; j++)
                 if(maze[i][j] == 'J') {
                     jr = i;
                     jc = j;
                     maze[i][j] = '.';
                 } else if(maze[i][j] == 'F') {
                     fires.push_back(Cell(i,j));
                     maze[i][j] = '.';
                 }
         }
         memset(vis, , sizeof(vis));
 
         // Joe
         vis[jr][jc][] = ;
         d[jr][jc][] = ;
         Q.push(Cell(jr, jc));
         bfs();
 
         // Fire
         for(int i = ; i < fires.size(); i++) {
             vis[fires[i].r][fires[i].c][] = ;
             d[fires[i].r][fires[i].c][] = ;
             Q.push(fires[i]);
         }
         bfs();
 
         // 计算答案
         ans = INF;
         for(int i = ; i < R; i++) {
             check(i,);
             check(i,C-);
         }
         for(int i = ; i < C; i++) {
             check(,i);
             check(R-,i);
         }
         if(ans == INF) printf("IMPOSSIBLE\n");
         else printf("%d\n", ans);
     }
     return ;
 }