Company Queries II

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

const int MAXN = 200001;
const int LOG = 18;

vector<int> adj[MAXN];
int up[MAXN][LOG]; // up[i][j] is the 2^j-th ancestor of i
int depth[MAXN];

void dfs(int v, int parent) {
    up[v][0] = parent; // the first ancestor (2^0) is the direct parent
    for (int i = 1; i < LOG; i++) {
        if (up[v][i - 1] != -1) {
            up[v][i] = up[up[v][i - 1]][i - 1]; // set the 2^i-th ancestor
        } else {
            up[v][i] = -1;
        }
    }

    for (int u : adj[v]) {
        if (u != parent) {
            depth[u] = depth[v] + 1;
            dfs(u, v);
        }
    }
}

int lca(int a, int b) {
    if (depth[a] < depth[b]) swap(a, b);

    // Bring both nodes to the same depth
    for (int i = LOG - 1; i >= 0; i--) {
        if (depth[a] - (1 << i) >= depth[b]) {
            a = up[a][i];
        }
    }

    if (a == b) return a;

    // Lift both nodes upwards until their lowest common ancestor is found
    for (int i = LOG - 1; i >= 0; i--) {
        if (up[a][i] != up[b][i]) {
            a = up[a][i];
            b = up[b][i];
        }
    }

    return up[a][0];
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int n, q;
    cin >> n >> q;

    for (int i = 2; i <= n; i++) {
        int boss;
        cin >> boss;
        adj[boss].push_back(i);
        adj[i].push_back(boss);
    }

    // Initialize DFS from the root (general director) employee 1
    depth[1] = 0;
    dfs(1, -1);

    // Process queries
    while (q--) {
        int a, b;
        cin >> a >> b;
        cout << lca(a, b) << "\n";
    }

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED