bzoj5152 [Wc2018]通道
正解：不会做。
写一个爬山算法就过官方数据了(逃
具体来说就是每次随机一个根，然后迭代找最长路的那个点。
多随机几次取$max$就行了。正解以后再补。。
1 #include <bits/stdc++.h> 2 #define il inline 3 #define RG register 4 #define ll long long 5 #define N (100005) 6 7 using namespace std; 8 9 int pos[N],vis[N],q[N],n; 10 ll dis[3][N],ans; 11 12 il int gi(){ 13 RG int x=0,q=1; RG char ch=getchar(); 14 while ((ch<'0' || ch>'9') && ch!='-') ch=getchar(); 15 if (ch=='-') q=-1,ch=getchar(); 16 while (ch>='0' && ch<='9') x=x*10+ch-'0',ch=getchar(); 17 return q*x; 18 } 19 20 struct graph{ 21 22 struct edge{ int nt,to; ll dis; }g[N<<1]; 23 24 int head[N],num; 25 26 il void insert(RG int from,RG int to,RG ll dis){ 27 g[++num]=(edge){head[from],to,dis},head[from]=num; return; 28 } 29 30 }G[3]; 31 32 il void bfs(RG int op,RG int S){ 33 for (RG int i=1;i<=n;++i) vis[i]=0; 34 RG int h=0,t=1; q[t]=S,vis[S]=1,dis[op][S]=0; 35 while (h<t){ 36 RG int x=q[++h],v; 37 for (RG int i=G[op].head[x];i;i=G[op].g[i].nt){ 38 v=G[op].g[i].to; if (vis[v]) continue; 39 vis[v]=1,q[++t]=v,dis[op][v]=dis[op][x]+G[op].g[i].dis; 40 } 41 } 42 return; 43 } 44 45 int main(){ 46 #ifndef ONLINE_JUDGE 47 freopen("tunnel.in","r",stdin); 48 freopen("tunnel.out","w",stdout); 49 #endif 50 srand(19260817),n=gi(); 51 for (RG int i=1,u,v;i<n;++i){ 52 u=gi(),v=gi(); RG ll w; scanf("%lld",&w); 53 G[0].insert(u,v,w),G[0].insert(v,u,w); 54 } 55 for (RG int i=1,u,v;i<n;++i){ 56 u=gi(),v=gi(); RG ll w; scanf("%lld",&w); 57 G[1].insert(u,v,w),G[1].insert(v,u,w); 58 } 59 for (RG int i=1,u,v;i<n;++i){ 60 u=gi(),v=gi(); RG ll w; scanf("%lld",&w); 61 G[2].insert(u,v,w),G[2].insert(v,u,w); 62 } 63 for (RG int i=1;i<=n;++i) pos[i]=i; 64 random_shuffle(pos+1,pos+n+1); 65 for (RG int T=1,rt;T<=25;++T){ 66 rt=pos[T]; RG ll ret=0; 67 while (1){ 68 bfs(0,rt),bfs(1,rt),bfs(2,rt); 69 RG ll res=0; RG int id=0; 70 for (RG int i=1;i<=n;++i) 71 if (res<dis[0][i]+dis[1][i]+dis[2][i]) 72 res=dis[0][i]+dis[1][i]+dis[2][i],id=i; 73 if (ret<res) ret=res,rt=id; else break; 74 } 75 ans=max(ans,ret); 76 } 77 cout<<ans; return 0; 78 }
