【Luogu】P2498 Save Princess Little Cloud (spfa)

　　topic link

　　I'm bursting...

　　It is easy to find that two circles can be connected with edges, the upper left is the starting point and the lower right is the end point, the minimum spanning tree is until the start point and the end point are connected, and the output edge weight / 2 is enough.

　　Then 80.

　　I don't understand why this can be converted into spfa to find the shortest path, and the adjacency matrix violently runs to AC.

#include<cstdio>
#include<algorithm>
#include<cctype>
#include<cstring>
#include<cstdlib>
#include<queue>
#include<cmath>
#define maxn 3030
using namespace std;
inline long long read(){
    long long num=0,f=1;
    char ch=getchar();
    while(!isdigit(ch)){
        if(ch=='-')    f=-1;
        ch=getchar();
    }
    while(isdigit(ch)){
        num = num * 10 + ch - 0 ' ;
        ch=getchar();
    }
    return num*f;
}

struct Node{    double x,y;    };
inline double calc(Node a,Node b){    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));    }
Node q[maxn];

struct Edge{
    int from,to;
    double val;
    bool operator <(const Edge a)const{
        return val<a.val;
    }
}d[maxn*maxn];
int cnt;

double dis[maxn];
bool vis[maxn];

double mp[maxn][maxn];

int main(){
    int e=read(),n=read(),m=read();
    int up=e+1,down=e+2,left=e+3,right=e+4;
    for(int i=1;i<=right;++i)
        for(int j=1;j<=right;++j)    mp[i][j]=0x7ffffffff;
    for(int i=1;i<=e;++i)    q[i]=(Node){read(),read()};
    for(int i=1;i<=e;++i){
        for(register int j=i+1;j<=e;++j)    mp[i][j]=mp[j][i]=calc(q[i],q[j]);
        mp[up][i]=(n-q[i].x)*2;
        mp[i][down]=(q[i].x-1)*2;
        mp[left][i]=(q[i].y-1)*2;
        mp[i][right]=(m-q[i].y)*2;
    }
    queue<int>f;    f.push(up);    f.push(left);
    for(int i=1;i<=right;++i)    dis[i]=0x7ffffffff;
    dis[up]=dis[left]=0;
    while(!f.empty()){
        int from=f.front();f.pop();    vis[from]=0;
        for(int to=1;to<=right;++to){
            if(max(mp[from][to],dis[from])<dis[to]){
                dis[to]=max(mp[from][to],dis[from]);
                if(vis[to])    continue;
                vis[to]=1;    f.push(to);
            }
        }
    }
    printf("%.2lf\n",min(dis[down],dis[right])/2.0);
    return 0;
}