lca.cpp

#include <bits/stdc++.h>
using namespace std;

#define ll long long int
#define X first
#define Y second

int n, LG;
vector<vector<int>> adj;

int timer;
vector<int> tin, tout;
vector<vector<int>> up;

void dfs(int v, int p)
{
    tin[v] = ++timer;
    up[v][0] = p;
    for (int i = 1; i <= LG; ++i)
        up[v][i] = up[up[v][i - 1]][i - 1];

    for (int u : adj[v])
    {
        if (u != p)
            dfs(u, v);
    }

    tout[v] = timer; // take care of this line
}

bool is_ancestor(int u, int v)
{
    return tin[u] <= tin[v] && tout[u] >= tout[v];
}

int lca(int u, int v)
{
    if (is_ancestor(u, v))
        return u;
    if (is_ancestor(v, u))
        return v;
    for (int i = LG; i >= 0; --i)
    {
        if (!is_ancestor(up[u][i], v))
            u = up[u][i];
    }
    return up[u][0];
}

void preprocess(int root)
{
    tin.resize(n + 1);
    tout.resize(n + 1);
    timer = 0;
    LG = ceil(log2(n)) + 1;
    up.assign(n, vector<int>(LG + 1));
    dfs(root, root);
}

void solve()
{
}

int main()
{
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
    int tc = 1;
    // cin>>tc;
    for (int i = 1; i <= tc; ++i)
    {
        solve();
    }
}