【SCOI2009】最长距离

题面

https://www.luogu.org/problem/P4162

题解

// luogu-judger-enable-o2
#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#define ri register int
#define N 33

using namespace std;

const int dx[]={1,-1,0,0},dy[]={0,0,-1,1};
char g[N][N];
int n,m,d;
int dis[N][N];

double dist(int x1,int y1,int x2,int y2){
  return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

struct node{
  int x,y;
  int s;
};
deque<node> q;

int main(){
  scanf("%d %d %d",&n,&m,&d);
  memset(g,'#',sizeof(g));
  for (ri i=1;i<=n;i++) {
    scanf("%s",g[i]+1);
    g[i][m+1]='#';
  }
  double ans=0;
  
  for (ri ii=1;ii<=n;ii++) 
    for (ri jj=1;jj<=m;jj++) {
      memset(dis,0x3f,sizeof(dis));
      int cc;
      if (g[ii][jj]=='1') cc=1; else cc=0;dis[ii][jj]=cc;
      q.push_back((node){ii,jj,cc});
      while (!q.empty()) {
        node cur=q.front(); q.pop_front();
        for (ri i=0;i<4;i++) {
          int nx=cur.x+dx[i],ny=cur.y+dy[i];
          if (g[nx][ny]=='#') continue;
          int cost;
          if (g[nx][ny]=='1') cost=1; else cost=0;
          if (dis[cur.x][cur.y]+cost<dis[nx][ny]) {
            dis[nx][ny]=dis[cur.x][cur.y]+cost;
            if (!cost) q.push_front((node){nx,ny,dis[nx][ny]}); 
            else q.push_back((node){nx,ny,dis[nx][ny]});
          }
        }
      }
      for (ri i=1;i<=n;i++) 
        for (ri j=1;j<=m;j++) if (dis[i][j]<=d) {
          if (dist(ii,jj,i,j)>ans) {
            //cout<<ii<<" "<<jj<<" "<<i<<" "<<j<<endl;
            ans=dist(ii,jj,i,j);
          }
        }
    }
    //cout<<ans<<endl;
    printf("%.6lf",ans);
}