[USACO09OPEN]捉迷藏Hide and Seek

OJ题号：洛谷2951

思路：Dijkstra+堆优化。注意是无向图，所以加边时要正反各加一遍。

#include<cstdio>
#include<vector>
#include<ext/pb_ds/priority_queue.hpp>
#define dis first
#define to second
const int N=20001,inf=0x7fffffff;
typedef std::pair<int,int> Edge;
int n,m,d[N];
std::vector<int> e[N];
void add(int a,int b) {
    e[a].push_back(b);
}
void dijkstra() {
    __gnu_pbds::priority_queue<Edge,std::greater<Edge> > q;
    __gnu_pbds::priority_queue<Edge,std::greater<Edge> >::point_iterator a[n+1];
    for(int i=1;i<=n;i++) {
        a[i]=q.push((i==1)?(Edge){0,i}:(Edge){inf,i});
        d[i]=(i==1)?0:inf;
    }
    while(!q.empty()) {
        Edge x=q.top();
        q.pop();
        if(x.dis==inf) continue;
        int u=x.to;
        for(std::vector<int>::iterator i=e[u].begin();i<e[u].end();i++) {
            int& next=*i;
            if(d[u]+1<d[next]) {
                d[next]=d[u]+1;
                q.modify(a[next],(Edge){d[next],next});
            }
        }
    }
}
int main() {
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++) {
        int a,b;
        scanf("%d%d",&a,&b);
        add(a,b);
        add(b,a);
    }
    dijkstra();
    int id,dis=0,count=0;
    for(int i=1;i<=n;i++) {
        if(d[i]==dis) count++;
        if(d[i]>dis) {
            id=i;
            dis=d[i];
            count=1;
        }
    }
    printf("%d %d %d
",id,dis,count);
    return 0;
}