[USACO09OPEN]捉迷藏Hide and Seek

Posted 2020-09-19 skylee的OI博客

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。

 1 #include<cstdio>
 2 #include<vector>
 3 #include<ext/pb_ds/priority_queue.hpp>
 4 #define dis first
 5 #define to second
 6 const int N=20001,inf=0x7fffffff;
 7 typedef std::pair<int,int> Edge;
 8 int n,m,d[N];
 9 std::vector<int> e[N];
10 void add(int a,int b) {
11     e[a].push_back(b);
12 }
13 void dijkstra() {
14     __gnu_pbds::priority_queue<Edge,std::greater<Edge> > q;
15     __gnu_pbds::priority_queue<Edge,std::greater<Edge> >::point_iterator a[n+1];
16     for(int i=1;i<=n;i++) {
17         a[i]=q.push((i==1)?(Edge){0,i}:(Edge){inf,i});
18         d[i]=(i==1)?0:inf;
19     }
20     while(!q.empty()) {
21         Edge x=q.top();
22         q.pop();
23         if(x.dis==inf) continue;
24         int u=x.to;
25         for(std::vector<int>::iterator i=e[u].begin();i<e[u].end();i++) {
26             int& next=*i;
27             if(d[u]+1<d[next]) {
28                 d[next]=d[u]+1;
29                 q.modify(a[next],(Edge){d[next],next});
30             }
31         }
32     }
33 }
34 int main() {
35     scanf("%d%d",&n,&m);
36     for(int i=1;i<=m;i++) {
37         int a,b;
38         scanf("%d%d",&a,&b);
39         add(a,b);
40         add(b,a);
41     }
42     dijkstra();
43     int id,dis=0,count=0;
44     for(int i=1;i<=n;i++) {
45         if(d[i]==dis) count++;
46         if(d[i]>dis) {
47             id=i;
48             dis=d[i];
49             count=1;
50         }
51     }
52     printf("%d %d %d\n",id,dis,count);
53     return 0;
54 }