[SCOI2009][BZOJ1295] 最长距离

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
#include<vector>
#define MOD 49999
#define INF 100000007
using namespace std;
double ans;
char s[31][31];
bool v[909];
int dx[5]={0,-1,0,1,0};
int dy[5]={0,0,-1,0,1};
int n,m,t,tot,edge,num[31][31],map[909][909],dis[909],q[50000],next[50000],head[909],list[50000],key[50000];
void insert(int x,int y,int z)
{
    next[++edge]=head[x];
    head[x]=edge;
    list[edge]=y;
    key[edge]=z;
}
void build()
{
    int xx,yy;
    for (int i=1;i<=n;i++)
        for (int j=1;j<=m;j++)
            for (int k=1;k<=4;k++)
            {
                xx=i+dx[k];
                yy=j+dy[k];
                if (xx<1||xx>n||yy<1||yy>m) continue;
                if (s[xx][yy-1]==‘1‘) insert(num[i][j],num[xx][yy],1); 
                else insert(num[i][j],num[xx][yy],0);
            }
}
void spfa(int sx,int sy)
{
    for (int i=1;i<=tot;i++) dis[i]=INF;
    memset(v,0,sizeof(v));
    v[num[sx][sy]]=1;
    dis[num[sx][sy]]=0;
    int x,t=0,w=1;q[1]=num[sx][sy];
    while (t!=w)
    {
        t=(t+1)%MOD;
        x=q[t];
        for (int i=head[x];i;i=next[i])
            if (dis[x]+key[i]<dis[list[i]])
            {
                dis[list[i]]=dis[x]+key[i];
                if (!v[list[i]])
                {    
                    w=(w+1)%MOD;
                    q[w]=list[i];
                    v[list[i]]=1;
                }
            }
        v[x]=0;
    }
    for (int i=1;i<=tot;i++) map[num[sx][sy]][i]=dis[i];
}    
double calc(int x,int y)
{
    int x1,x2,y1,y2,sum;
    x1=(x-1)/m+1; y1=(x-1)%m+1;
    x2=(y-1)/m+1; y2=(y-1)%m+1;
    sum=(x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
    return sqrt(double(sum));
}
int main()
{
    scanf("%d%d%d",&n,&m,&t);
    for (int i=1;i<=n;i++)
        scanf("%s",s[i]);
    tot=0; edge=0;
    for (int i=1;i<=n;i++)
         for (int j=1;j<=m;j++) 
             num[i][j]=++tot;
     build();
     for (int i=1;i<=n;i++)
         for (int j=1;j<=m;j++)
             spfa(i,j);
     for (int i=1;i<=tot;i++) 
         for (int j=1;j<=tot;j++)
         {
            int x1=(i-1)/m+1,y1=(i-1)%m+1;
            if (map[i][j]<=t-(s[x1][y1-1]==‘1‘))
            {
                double x=calc(i,j);
                ans=max(ans,x);
            }
        }
    printf("%.6lf",ans);
    return 0;
}