Pikachu And Champions - LCA

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

const int MAX_SIZE = 500010;
const int MAX_LEVELS =log2(MAX_SIZE) + 1;

int N, Q;
vector<vector<int>> graph;

int depth[MAX_SIZE];
int parents[MAX_SIZE][MAX_LEVELS];

void dfs(int node, int parent) {
    parents[node][0] = parent;
    for (int i: graph[node]) {
        if (i != parent) {
            depth[i] = depth[node] + 1;
            dfs(i, node);
        }
    }
}
int lca(int u, int v) {
    if (depth[u] < depth[v]) {
        swap(u, v);
    }
    for (int i = MAX_LEVELS - 1; i >= 0; i --) {
        if (depth[u] >= depth[v] + (1 << i)) {
            u = parents[u][i];
        }
    }
    if (u == v) {
        return u;
    }
    for (int i = MAX_LEVELS - 1; i >= 0; i --) {
        if (parents[u][i] != 0 && parents[u][i] != parents[v][i]) {
            u = parents[u][i];
            v = parents[v][i];
        }
    }
    return parents[u][0];
}

int main(int argc, const char * argv[]) {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> N;
    int m;
    cin >> m;

    graph.resize(N + 1);
    for (int i = 1; i < N; i ++) {
        int u, v;
        cin >> u >> v;
        graph[u].push_back(v);
        graph[v].push_back(u);
    }
    dfs(1, 0);
    for (int i = 1; i < MAX_LEVELS; i ++) {
        for (int j = 1; j <= N; j ++) {
            if (parents[j][i - 1] != 0) {
                parents[j][i] = parents[parents[j][i - 1]][i - 1];
            }
        }
    }
    for(int i=0;i<m;i++)
    {
        int k;cin >> k;
        int mx = -1;
        int a[k];
        for(int i=0;i<k;i++)
        {
            cin >> a[i];
        }
        int m = 0;
        int id =0;
        for(int i=1;i<k;i++)
        {
            int p = depth[a[i]]+depth[a[0]] - 2*depth[lca(a[i],a[0])];
            if(p>m)
            {
                m=p;id=i;
            }
        }
        for(int i=0;i<k;i++)
        {
            int p = depth[a[i]]+depth[a[id]] - 2*depth[lca(a[i],a[id])];
            mx = max(mx,p);
        }
        cout << mx << '\n';
    }
    return 0;
}