#include<cstring>
#include<cstdlib>
#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn = 5e5 + 10;
const int DEG = 20;
struct node
{
int to, next;
};
node edge[maxn<<1];
int head[maxn];
int fa[maxn][20];
int deep[maxn];
int tot;
void add(int u, int v)
{
edge[tot].to = v;
edge[tot].next = head[u];
head[u] = tot++;
}
void dfs(int u, int f, int dep)
{
fa[u][0] = f;
deep[u] = dep;
for(int i = head[u]; ~i; i = edge[i].next)
{
int v = edge[i].to;
if(v == f)
continue;
dfs(v, u, dep + 1);
}
}
int lca(int u, int v)
{
if(deep[u] > deep[v])
swap(u, v);
for(int dep = deep[v] - deep[u], i = 0; dep;dep>>= 1, i++)
if(dep & 1)
v = fa[v][i];
if(u == v)
return u;
for(int i = DEG - 1; i >= 0; i--)
{
if(fa[u][i] == fa[v][i])
continue;
u = fa[u][i];
v = fa[v][i];
}
return fa[u][0];
}
int main()
{
int n, m, s;
tot = 0;
scanf("%d%d%d", &n, &m, &s);
memset(head, -1, sizeof(head));
for(int i = 0; i < n - 1; ++i)
{
int u, v;
scanf("%d%d", &u, &v);
add(u, v);
add(v, u);
}
dfs(s, 0, 1);
for(int i = 1; i < DEG; i++)
{
for(int j = 1; j <= n; j++)
fa[j][i] = fa[fa[j][i - 1]][i - 1];
}
while(m--)
{
int u, v;
scanf("%d%d", &u, &v);
printf("%d\n", lca(u, v));
}
return 0;
}
P3379 [template] recent common ancestor (LCA) multiplied template
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Origin blog.csdn.net/weixin_43891021/article/details/100080293
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