洛谷 1821 [USACO07FEB]银牛派对Silver Cow Party

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #define LL long long
 5 #define rg register
 6 #define N 200010
 7 using namespace std;
 8 int n,m,s,ans,tot,last[N],dis[N],dis2[N],pos[N];
 9 struct rec{
10     int u,v,d;
11 }r[N<<1];
12 struct edge{
13     int to,pre,dis;
14 }e[N<<1];
15 struct heap{
16     int poi,dis;
17 }h[N];
18 inline int read(){
19     int k=0,f=1; char c=getchar();
20     while(c<‘0‘||c>‘9‘)c==‘-‘&&(f=-1),c=getchar();
21     while(‘0‘<=c&&c<=‘9‘)k=k*10+c-‘0‘,c=getchar();
22     return k*f;
23 }
24 inline void up(int x){
25     int fa;
26     while((fa=(x>>1))&&h[fa].dis>h[x].dis){
27         swap(h[fa],h[x]); swap(pos[h[fa].poi],pos[h[x].poi]);
28         x=fa;
29     }
30 }
31 inline void down(int x){
32     int son;
33     while((son=(x<<1))<=tot){
34         if(son<tot&&h[son].dis>h[son+1].dis) son++;
35         if(h[son].dis<h[x].dis){
36             swap(h[son],h[x]); swap(pos[h[son].poi],pos[h[x].poi]);
37             x=son;
38         }
39         else return;
40     }
41 }
42 inline void dijkstra(int x){
43     for(rg int i=1;i<=n*2;i++) dis[i]=1e9;
44     h[tot=pos[x]=1]=(heap){x,dis[x]=0};
45     while(tot){
46         int now=h[1].poi; h[1]=h[tot--]; if(tot) down(1);
47         for(rg int i=last[now],to;i;i=e[i].pre)
48         if(dis[to=e[i].to]>dis[now]+e[i].dis){
49             dis[to]=dis[now]+e[i].dis;
50             if(!pos[to]) h[pos[to]=++tot]=(heap){to,dis[to]};
51             else h[pos[to]].dis=dis[to];
52             up(pos[to]);
53         }
54         pos[now]=0;
55     }
56 }
57 int main(){
58     n=read(); m=read(); s=read();
59     for(rg int i=1;i<=m;i++){
60         int u=read(),v=read(),d=read();
61         r[i]=(rec){u,v,d};
62         e[++tot]=(edge){v,last[u],d}; last[u]=tot;
63     }
64     dijkstra(s);
65     for(rg int i=1;i<=n;i++) dis2[i]=dis[i];
66     memset(last,0,sizeof(last));
67     for(rg int i=1;i<=m;i++){
68         int u=r[i].u,v=r[i].v,d=r[i].d;
69         e[++tot]=(edge){u,last[v],d}; last[v]=tot; 
70     }
71     dijkstra(s);
72     for(rg int i=1;i<=n;i++) ans=max(ans,dis[i]+dis2[i]);
73     printf("%d\n",ans);
74     return 0;
75 }