[BZOJ2709][Violet 1]迷宫花园（二分+spfa）

题目描述

题解

代码

#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
#define N 50005

const double eps=1e-7;
char str[105][105];
double L,ans;int T,n,m,s,t;
int tot,point
,nxt
,v
;double c
;bool flag
;
double dis
;bool vis
;
queue <int> q;

void clear()
{
L=ans=0;n=m=s=t=0;
tot=0;memset(point,0,sizeof(point));
memset(str,0,sizeof(str));
}
double spfa(int s,int t)
{
memset(dis,127,sizeof(dis));dis[s]=0;
memset(vis,0,sizeof(vis));vis[s]=1;
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]+c[i])
{
dis[v[i]]=dis[now]+c[i];
if (!vis[v[i]]) vis[v[i]]=1,q.push(v[i]);
}
}
return dis[t];
}
double check(double mid)
{
for (int i=1;i<=tot;++i)
if (flag[i]) c[i]=mid;
return spfa(s,t);
}
double find()
{
double l=0,r=10,mid,ans;
while (r-l>eps)
{
mid=(l+r)/2;
double dist=check(mid);
if (dist<L) ans=l=mid;
else r=mid;
}
return ans;
}
void add(int x,int y,double z,int opt)
{
++tot; nxt[tot]=point[x]; point[x]=tot; v[tot]=y; c[tot]=z; flag[tot]=opt;
}
int main()
{
scanf("%d",&T);
while (T--)
{
clear();
scanf("%lf",&L);
scanf("%d%d\n",&n,&m);
for (int i=1;i<=n;++i)
{
gets(str[i]+1);
for (int j=1;j<=m;++j)
if (str[i][j]=='S') s=(i-1)*m+j;
else if (str[i][j]=='E') t=(i-1)*m+j;
}
for (int i=1;i<n;++i)
for (int j=1;j<m;++j)
if (str[i][j]=='S'||str[i][j]=='E'||str[i][j]==' ')
{
int now=(i-1)*m+j;
if (str[i][j+1]=='S'||str[i][j+1]=='E'||str[i][j+1]==' ') add(now,now+1,1,0),add(now+1,now,1,0);
if (str[i+1][j]=='S'||str[i+1][j]=='E'||str[i+1][j]==' ') add(now,now+m,0,1),add(now+m,now,0,1);
}
ans=find();
printf("%.5lf\n",ans);
}
}