P3379 【模板】最近公共祖先（LCA）倍增模板

#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;

}