poj3268

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstdlib>
 4 #include<algorithm>
 5 #include<vector>
 6 #include<queue>
 7 #include<cmath>
 8 
 9 using namespace std;
10 
11 const int MAX_V=1001;
12 const int MAX_E=100001;
13 const int INF=99999999;
14 
15 typedef pair<int,int> P;
16 
17 struct edge{
18     int from,to,cost;
19 };
20 
21 int d1[MAX_V];//从起点出发到任意一点的最短路
22 int d2[MAX_V];//从任一点出发到起点的最短路
23 
24 vector<edge>es1[MAX_E];
25 vector<edge>es2[MAX_E];
26 
27 //就是求单源最短路的
28 void dijkstra(int s,int* d,vector<edge> es[]){
29     fill(d,d+MAX_V,INF);
30     d[s]=0;
31     priority_queue<P,vector<P>,greater<P> > que;
32     que.push(P(0,s));
33     while(!que.empty()){
34         P p=que.top();que.pop();
35         int v=p.second;
36         if(d[v]<p.first)continue;
37         for(int i=0;i<es[v].size();i++){
38             edge e=es[v][i];
39             if(d[e.to]>d[e.from]+e.cost){
40                 d[e.to]=d[e.from]+e.cost;
41                 que.push(P(d[e.to],e.to));
42             }
43         }
44     }
45     
46 }
47 
48 int main() {
49     int n,m,x;
50     cin>>n>>m>>x;
51     while(m--){
52         edge temp;
53         scanf("%d%d%d",&temp.from,&temp.to,&temp.cost);
54         es1[temp.from].push_back(temp);
55         swap(temp.from,temp.to);
56         es2[temp.from].push_back(temp);
57     }
58     int ans=-1;
59     for(int i=1;i<=n;i++){
60         dijkstra(i,d1,es1);//从起点出发到任意一点的最短路
61         dijkstra(i,d2,es2);//从任一点出发到起点的最短路
62         ans=max(ans,d1[x]+d2[x]);
63     }
64     cout<<ans<<endl;
65     return 0;
66 }