二分+LCA+查分前缀和
1 #include<iostream> 2 #include<cstring> 3 #include<cstdio> 4 #include<algorithm> 5 using namespace std; 6 int read() 7 { 8 int x=0,f=1;char ch;ch=getchar(); 9 while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();} 10 while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();} 11 return x*f; 12 } 13 struct data{int to,next,v,from;}e[600001]; 14 struct data2{int to,next,from,num;}as[600001]; 15 int lca[300001],dis[300001]; 16 int head[300001],head2[300001],cnt; 17 void add(int u,int v,int w){e[cnt].from=u,e[cnt].to=v,e[cnt].next=head[u],e[cnt].v=w,head[u]=cnt;cnt++;} 18 void add2(int u,int v,int nu){as[cnt].to=v,as[cnt].next=head2[u],as[cnt].from=u,as[cnt].num=nu,head2[u]=cnt,cnt++;} 19 int n,m,l=0,r=0; 20 int d[300001]; 21 bool vis[300001]; 22 int fa[300001]; 23 int p[300001]; 24 int s[300001],t1[300001]; 25 int find(int x){return x==fa[x]?fa[x]:fa[x]=find(fa[x]);} 26 int maxn=0,maxj=0; 27 void dfs(int now,int d) 28 { 29 dis[now]=d; 30 vis[now]=1; 31 for(int i=head[now];i>=0;i=e[i].next) if(!vis[e[i].to]) dfs(e[i].to,d+e[i].v); 32 } 33 void Lca(int now) 34 { 35 vis[now]=1; 36 for(int i=head[now];i>=0;i=e[i].next) 37 if(!vis[e[i].to]) 38 { 39 Lca(e[i].to);fa[find(e[i].to)]=find(now); 40 } 41 for(int i=head2[now];i>=0;i=as[i].next) 42 { 43 if(vis[as[i].to]) 44 { 45 lca[as[i].num]=find(as[i].to); 46 //cout<<dis[as[i].from]<<' '<<dis[as[i].to]<<' '<<2*dis[lca[as[i].num]]<<endl; 47 d[as[i].num]=abs(dis[as[i].from]+dis[as[i].to]-2*dis[lca[as[i].num]]); 48 r=max(r,d[as[i].num]); 49 } 50 } 51 } 52 int work(int now,int t,int from) 53 { 54 int sum=p[now]; 55 for(int i=head[now];i>=0;i=e[i].next) 56 if(e[i].to!=from) sum+=work(e[i].to,t,e[i].from); 57 if(sum==t)maxj=max(maxj,dis[now]-dis[from]); 58 return sum; 59 } 60 bool check(int mid,int t) 61 { 62 work(1,t,0); 63 if(maxn-maxj<=mid) return 1; 64 else return 0; 65 } 66 int main() 67 { 68 memset(head,-1,sizeof(head)); 69 memset(head2,-1,sizeof(head2)); 70 n=read(),m=read(); 71 for(int i=1;i<=n;i++) fa[i]=i; 72 for(int i=1;i<n;i++) 73 { 74 int u,v,w; 75 u=read(),v=read(),w=read(); 76 add(u,v,w); 77 add(v,u,w); 78 } 79 cnt=0; 80 for(int i=1;i<=m;i++) 81 { 82 s[i]=read(),t1[i]=read(); 83 add2(s[i],t1[i],i); 84 add2(t1[i],s[i],i); 85 } 86 dfs(1,0); 87 memset(vis,0,sizeof(vis)); 88 Lca(1); 89 //for(int i=1;i<=n;i++) cout<<dis[i]<<endl; 90 //for(int i=1;i<=m;i++) cout<<d[i]<<endl; 91 while(l<=r) 92 { 93 int t=0; 94 int mid=(l+r)>>1; 95 maxj=0; 96 memset(p,0,sizeof(p)); 97 for(int i=1;i<=m;i++) 98 { 99 if(d[i]>mid){maxn=max(maxn,d[i]);t++;p[s[i]]++,p[t1[i]]++,p[lca[i]]-=2;} 100 } 101 if(check(mid,t)) r=mid-1; 102 else l=mid+1; 103 } 104 cout<<l; 105 return 0; 106 }