在树上建主席树......
每个点从父亲那里建过来，最后建出来就是从根到$i$这条链上的主席树，查询的时候一边差分一边查询
($cmt[u]+cmt[v]-cmt[lca(u,v)]-cmt[anc[lca(u,v)]]$)
1 #include<cstdio> 2 #include<cstring> 3 #include<algorithm> 4 using namespace std; 5 const int N=100005,K=18; 6 int p[N],noww[2*N],goal[2*N]; 7 int siz[N],anc[N],dep[N],imp[N],top[N]; 8 int num[N],tep[N],cmt[N*K],son[N*K][2],sum[N*K],uni[N*K]; 9 int n,m,t1,t2,t3,lca,cnt,tot,len,xnt,ans,n1,n2,n3,n4; 10 void link(int f,int t) 11 { 12 noww[++cnt]=p[f]; 13 goal[cnt]=t,p[f]=cnt; 14 } 15 int Create(int pre,int l,int r,int pos) 16 { 17 int nde=++xnt; 18 son[nde][0]=son[pre][0]; 19 son[nde][1]=son[pre][1]; 20 sum[nde]=sum[pre]+1; 21 if(l<r) 22 { 23 int mid=(l+r)/2; 24 if(pos<=mid) son[nde][0]=Create(son[pre][0],l,mid,pos); 25 else son[nde][1]=Create(son[pre][1],mid+1,r,pos); 26 } 27 return nde; 28 } 29 int Query(int l,int r,int rnk) 30 { 31 if(l==r) return l; 32 int mid=(l+r)/2,noww=sum[son[n1][0]]; 33 noww+=sum[son[n2][0]]-sum[son[n3][0]]-sum[son[n4][0]]; 34 if(rnk<=noww) 35 { 36 n1=son[n1][0],n2=son[n2][0],n3=son[n3][0],n4=son[n4][0]; 37 return Query(l,mid,rnk); 38 } 39 else 40 { 41 n1=son[n1][1],n2=son[n2][1],n3=son[n3][1],n4=son[n4][1]; 42 return Query(mid+1,r,rnk-noww); 43 } 44 } 45 void DFS(int nde,int fth,int dth) 46 { 47 int tmp=0; 48 siz[nde]=1,dep[nde]=dth,anc[nde]=fth; 49 cmt[nde]=Create(cmt[anc[nde]],1,len,num[nde]); 50 for(int i=p[nde];i;i=noww[i]) 51 if(goal[i]!=fth) 52 { 53 DFS(goal[i],nde,dth+1); 54 siz[nde]+=siz[goal[i]]; 55 if(siz[goal[i]]>tmp) 56 tmp=siz[goal[i]],imp[nde]=goal[i]; 57 } 58 } 59 void MARK(int nde,int tpp) 60 { 61 top[nde]=tpp; 62 if(imp[nde]) 63 { 64 MARK(imp[nde],tpp); 65 for(int i=p[nde];i;i=noww[i]) 66 if(goal[i]!=anc[nde]&&goal[i]!=imp[nde]) 67 MARK(goal[i],goal[i]); 68 } 69 } 70 int LCA(int x,int y) 71 { 72 while(top[x]!=top[y]) 73 { 74 if(dep[top[x]]<dep[top[y]]) 75 swap(x,y); x=anc[top[x]]; 76 } 77 return dep[x]<dep[y]?x:y; 78 } 79 int main () 80 { 81 scanf("%d%d",&n,&m); 82 for(int i=1;i<=n;i++) 83 scanf("%d",&num[i]),tep[i]=num[i]; 84 sort(tep+1,tep+1+n),len=unique(tep+1,tep+1+n)-tep-1; 85 for(int i=1;i<=n;i++) 86 num[i]=lower_bound(tep+1,tep+1+len,num[i])-tep; 87 for(int i=1;i<n;i++) 88 scanf("%d%d",&t1,&t2),link(t1,t2),link(t2,t1); 89 DFS(1,0,1); MARK(1,1); 90 for(int i=1;i<=m;i++) 91 { 92 scanf("%d%d%d",&t1,&t2,&t3),t1^=ans,lca=LCA(t1,t2); 93 n1=cmt[t1],n2=cmt[t2],n3=cmt[lca],n4=cmt[anc[lca]]; 94 printf("%d ",ans=tep[Query(1,len,t3)]); 95 } 96 return 0; 97 }