Orz
#include<cstdio> #include<iostream> #include<cstring> #include<algorithm> #define maxn 500010 using namespace std; struct node { int ed,nxt; }; node edge[maxn<<1]; int n,m,first[maxn],cnt,root; int dep[maxn],fa[maxn],top[maxn],rnk[maxn],son[maxn],siz[maxn],rnk_cnt; inline void add_edge(int s,int e) { ++cnt; edge[cnt].ed=e; edge[cnt].nxt=first[s]; first[s]=cnt; return; } inline void dfs_1(int now,int pre) { dep[now]=dep[pre]+1; fa[now]=pre; siz[now]=1; for(register int i=first[now];i;i=edge[i].nxt) { int e=edge[i].ed; if(e==fa[now]) continue; dfs_1(e,now); siz[now]+=siz[e]; if(son[now]==-1||siz[e]>siz[son[now]]) son[now]=e; } return; } inline void dfs_2(int now,int heavy_fa) { top[now]=heavy_fa; if(son[now]==-1) return; else dfs_2(son[now],heavy_fa); for(register int i=first[now];i;i=edge[i].nxt) { int e=edge[i].ed; if(e!=fa[now]&&e!=son[now]) dfs_2(e,e); } return; } inline 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]]; } if(dep[x]>dep[y]) swap(x,y); return x; } int main() { scanf("%d%d%d",&n,&m,&root); for(register int i=1;i<=n-1;++i) { int s,e; scanf("%d%d",&s,&e); add_edge(s,e); add_edge(e,s); } memset(son,-1,sizeof(son)); dfs_1(root,0); dfs_2(root,root); for(register int i=1;i<=m;++i) { int x,y; scanf("%d%d",&x,&y); printf("%d\n",lca(x,y)); } return 0; } 来源：博客园
作者：Hoyoak
链接：https://www.cnblogs.com/Hoyoak/p/11813542.html