思路：

兩次BFS

一次處理火燒，得到每個點著火的時間，第二次bfs處理人的逃跑。

注意，像這類有多組資料輸入的時候一定要注意初始化條件，比如幾個vis陣列要置零，佇列一定要記得清空。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
int sx,sy,tx,ty;
int n,m,k;
struct node{
	int x,y,time;
}nextt;
char mp[105][105];
int vis1[105][105];
int vis2[105][105];
int moves[8][2]={{1,0},{1,1},{1,-1},{0,1},{0,-1},{-1,1},{-1,0},{-1,-1}};
int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
int mpt[105][105];//火燒時間 
queue<node>q;
//bfs火燒，預處理時間 
void bfs_fire(){
	while(!q.empty()){
		node temp=q.front();
		q.pop();
		for(int i=0;i<8;i++){
			nextt.x=temp.x+moves[i][0];
			nextt.y=temp.y+moves[i][1];
			nextt.time=temp.time+k;
			if(!vis1[nextt.x][nextt.y]&&nextt.x>=0&&nextt.y>=0&&nextt.x<n&&nextt.y<m){
				mpt[nextt.x][nextt.y]=nextt.time;
				vis1[nextt.x][nextt.y]=1;
				q.push(nextt);
			}
		}
	}
}
//人逃跑 
void bfs_run(){
	while(!q.empty()) q.pop();
	q.push((node){sx,sy,0});
	vis2[sx][sy]=1;
	while(!q.empty()){
		node temp=q.front();
		q.pop();
		if(temp.x==tx&&temp.y==ty){
			printf("%d\n",temp.time);
			return ;
		}
		for(int i=0;i<4;i++){
			nextt.x=temp.x+dx[i];
			nextt.y=temp.y+dy[i];
			nextt.time=temp.time+1;
			if(!vis2[nextt.x][nextt.y]&&nextt.x>=0&&nextt.y>=0&&nextt.x<n&&nextt.y<m){
				if(mpt[nextt.x][nextt.y]>nextt.time){
					
					vis2[nextt.x][nextt.y]=1;
					q.push(nextt);
				}
			}
		}
	}
	printf("Impossible\n");
	return ;
}
int main(){
	while(scanf("%d%d%d",&n,&m,&k)){
		if(n==0||m==0||k==0) break;
		memset(vis1,0,sizeof(vis1));
		memset(mp,0,sizeof(mp));
		memset(mpt,0,sizeof(mpt));
		memset(vis2,0,sizeof(vis2));
		while(!q.empty()) q.pop();
		for(int i=0;i<n;i++){
			scanf("%s",mp[i]);
		}
		for(int i=0;i<n;i++){
			for(int j=0;j<m;j++){
				if(mp[i][j]=='f'){
					q.push((node){i,j,0});
					vis1[i][j]=1; 
				}
			}
		}
		for(int i=0;i<n;i++){
			for(int j=0;j<m;j++){
				if(mp[i][j]=='s'){
					sx=i,sy=j;
				}
				if(mp[i][j]=='t'){
					tx=i,ty=j;
				}
			}
		}
		if(q.empty()) printf("%d\n",abs(tx-sx)+abs(ty-sy));
		else{
		bfs_fire();
		bfs_run();
		}
	}
	return 0;
}