[bzoj2709][Violet 1]迷宫花园（二分+spfa）

题目：

题解：

代码：

#include <cstdio>
#include <iostream>
#include <cstring>
#include <queue>
#define N 40050
using namespace std;
const double eps=1e-7;
int n,m,s,t;bool vis
,chan
;
int tot,nxt
,point
,v
;double cc
,dis
,L;
char st[105][105];
void addline(int x,int y,double z,int fl)
{
++tot; nxt[tot]=point[x]; point[x]=tot; v[tot]=y; cc[tot]=z; chan[tot]=fl;
++tot; nxt[tot]=point[y]; point[y]=tot; v[tot]=x; cc[tot]=z; chan[tot]=fl;
}
bool spfa(double mid)
{
queue<int>q;
for (int i=1;i<=tot;i++) if (chan[i]) cc[i]=mid;
memset(vis,0,sizeof(vis));
memset(dis,0x7f,sizeof(dis));
dis[s]=0;
q.push(s);
while (!q.empty())
{
int now=q.front(); q.pop(); vis[now]=0;
for (int i=point[now];i;i=nxt[i])
if (dis[v[i]]>dis[now]+cc[i])
{
dis[v[i]]=dis[now]+cc[i];
if (!vis[v[i]]) vis[v[i]]=1,q.push(v[i]);
}
}
if (dis[t]<=L) return 1;
return 0;
}
int main()
{
int T,i,j;
scanf("%d",&T);
while (T--)
{
tot=0; memset(point,0,sizeof(point));memset(chan,0,sizeof(chan));
scanf("%lf%d%d\n",&L,&n,&m);
for (i=1;i<=n;i++)
{
gets(st[i]+1);
for (j=1;j<=m;j++)
if (st[i][j]=='S') s=(i-1)*m+j;
else if (st[i][j]=='E') t=(i-1)*m+j;
}
for (i=1;i<=n;i++)
for (j=1;j<=m;j++)
if (st[i][j]==' '||st[i][j]=='E'||st[i][j]=='S')
{
int now=(i-1)*m+j;
if (j+1<=m && (st[i][j+1]==' '||st[i][j+1]=='E'||st[i][j+1]=='S'))
addline(now,now+1,1,0);
if (i+1<=n && (st[i+1][j]==' '||st[i+1][j]=='E'||st[i+1][j]=='S'))
addline(now,now+m,0,1);
}
double l=0,r=10;
while (r-l>=eps)
{
double mid=(l+r)/2.0;
if (spfa(mid)) l=mid;
else r=mid;
}
printf("%.5lf\n",l);
}
}