//LCA //树链剖分 在线 #include<iostream> #include<cstdio> #include<cstring> #include<cstdlib> #include<cmath> #include<algorithm> using namespace std; int n,m,root,cnt,head[500001]; int hvyson[500001],fa[500001],siz[500001],dep[500001],top[500001]; struct uio{ int to,next; }edge[1000001]; void add(int x,int y) { edge[++cnt].next=head[x]; edge[cnt].to=y; head[x]=cnt; } void dfs1(int x,int f,int depth) { dep[x]=depth; fa[x]=f; siz[x]=1; int maxson=-1; for(int i=head[x];i;i=edge[i].next) { int y=edge[i].to; if(y==f) continue; dfs1(y,x,depth+1); siz[x]+=siz[y]; if(siz[y]>maxson) { hvyson[x]=y; maxson=siz[y]; } } } void dfs2(int x,int topf) { top[x]=topf; if(!hvyson[x]) return; dfs2(hvyson[x],topf); for(int i=head[x];i;i=edge[i].next) { int y=edge[i].to; if(y==fa[x]||y==hvyson[x]) continue; dfs2(y,y); } } int lca(int x,int y) { while(top[x]!=top[y]) { if(dep[top[x]]<dep[top[y]]) swap(x,y); x=fa[top[x]]; } return dep[x]<dep[y]? x : y; } void do_something() { for(int i=1;i<=m;i++) { int u,v; scanf("%d%d",&u,&v); printf("%d\n",lca(u,v)); } } int main() { scanf("%d%d%d",&n,&m,&root); for(int i=1;i<n;i++) { int u,v; scanf("%d%d",&u,&v); add(u,v); add(v,u); } dfs1(root,0,1); dfs2(root,root); do_something(); return 0; }
原文：https://www.cnblogs.com/water-radish/p/9280519.html