比赛的时候我被分配到这一题，然后导致学长的队伍少过一题......，心怀内疚 $qwq$
所以我就去学了 $LCT$ 维护直径， $LCT$ 维护直径我上一个博客讲得很详细了：传送门
这里维护虚儿子用的是 $multiset$ ，没写可删堆
#include<iostream> #include<cstdio> #include<algorithm> #include<cstring> #include<cmath> #include<set> #include<queue> using namespace std; typedef long long ll; inline int read() { int x=0,f=1; char ch=getchar(); while(ch<'0'||ch>'9') { if(ch=='-') f=-1; ch=getchar(); } while(ch>='0'&&ch<='9') { x=(x<<1)+(x<<3)+(ch^48); ch=getchar(); } return x*f; } const int N=8e5+7; const ll INF=1e18; int n,m; struct edge { int x,y; }e[N]; namespace LCT { int c[N][2],fa[N],sz[N],val[N]; ll sum[N],lmx[N],rmx[N],mxs[N]; bool rev[N]; multiset <ll> H[N],P[N]; inline void ins(int u,int v) { H[u].insert(lmx[v]); P[u].insert(mxs[v]); } inline void del(int u,int v) { H[u].erase(H[u].find(lmx[v])); P[u].erase(P[u].find(mxs[v])); } inline ll fir(multiset <ll> &S) { return S.size() ? *S.rbegin() : -INF; } inline ll sec(multiset <ll> &S) { return S.size()>1 ? *(++S.rbegin()) : -INF; } inline void pushup(int x) { int &lc=c[x][0],&rc=c[x][1]; sum[x]=sum[lc]+sum[rc]+val[x]; ll t=max(0ll,fir(H[x])), L=max(t,rmx[lc])+val[x], R=max(t,lmx[rc])+val[x]; lmx[x]=max(lmx[lc], sum[lc]+R ); rmx[x]=max(rmx[rc], sum[rc]+L ); mxs[x]=max( max( rmx[lc]+R , lmx[rc]+L ) , max(mxs[lc],mxs[rc]) ); mxs[x]=max(mxs[x],fir(P[x])); mxs[x]=max(mxs[x], t+val[x] ); mxs[x]=max(mxs[x], t+val[x]+sec(H[x]) ); } inline void pushdown(int x) { if(!x||!rev[x]) return; int &lc=c[x][0],&rc=c[x][1]; rev[x]=0; swap(lc,rc); swap(lmx[x],rmx[x]); if(lc) rev[lc]^=1; if(rc) rev[rc]^=1; } inline void rever(int x) { rev[x]=1; pushdown(x); } inline bool noroot(int x) { return (c[fa[x]][0]==x)|(c[fa[x]][1]==x); } inline void rotate(int x) { int y=fa[x],z=fa[y],d=(c[y][1]==x); if(noroot(y)) c[z][c[z][1]==y]=x; fa[x]=z; fa[y]=x; fa[c[x][d^1]]=y; c[y][d]=c[x][d^1]; c[x][d^1]=y; pushup(y); } void push_rev(int x) { if(noroot(x)) push_rev(fa[x]); else pushdown(x); pushdown(c[x][0]); pushdown(c[x][1]); } inline void splay(int x) { push_rev(x); while(noroot(x)) { int y=fa[x],z=fa[y]; if(noroot(y)) (c[y][0]==x ^ c[z][0]==y) ? rotate(x) : rotate(y); rotate(x); } pushup(x); } inline void access(int x) { for(int y=0;x;y=x,x=fa[x]) { splay(x); if(y) del(x,y); if(c[x][1]) ins(x,c[x][1]); c[x][1]=y; pushup(x); } } inline void makeroot(int x) { access(x); splay(x); rever(x); } inline int findroot(int x) { access(x); splay(x); pushdown(x); while(c[x][0]) x=c[x][0],pushdown(x); splay(x); return x; } inline void split(int x,int y) { makeroot(x); access(y); splay(y); } inline void link(int x,int y) { makeroot(x); if(findroot(y)==x) return; makeroot(y); fa[x]=y; ins(y,x); pushup(y); } inline void cut(int x,int y) { makeroot(x); if(findroot(y)!=x||fa[y]!=x||c[y][0]) return; fa[y]=c[x][1]=0; pushup(x); } inline void Link(int k) { link(e[k].x,n+k); link(e[k].y,n+k); } inline void Cut(int k) { cut(e[k].x,n+k); cut(e[k].y,n+k); } inline ll query(int x) { access(x); splay(x); return rmx[x]; } } int main() { n=read(); for(int i=1;i<n;i++) e[i].x=read(),e[i].y=read(),LCT::val[n+i]=read(); for(int i=1;i<n;i++) LCT::Link(i); m=read(); char s[7]; int a; for(int i=1;i<=m;i++) { scanf("%s",s); a=read(); if(s[0]=='Q') printf("%lld\n",LCT::query(a)); else LCT::Cut(a),LCT::val[n+a]=read(),LCT::Link(a); } return 0; }