Cod sursa(job #2636817)

Utilizator vlad082002Ciocoiu Vlad vlad082002 Data 20 iulie 2020 11:06:08
Problema Lowest Common Ancestor Scor 100
Compilator cpp-64 Status done
Runda Arhiva educationala Marime 1.49 kb
#include <fstream>
#include <vector>
using namespace std;

ifstream fin("lca.in");
ofstream fout("lca.out");

int rmq[20][200005], n, m, prima[100005], lg[200005];
vector <int> euler, nivel, g[100005];

void citire() {
    fin >> n >> m;
    for(int i = 2; i <= n; i++) {
        int x;
        fin >> x;
        g[x].push_back(i);
    }
}

void dfs(int x, int niv) {
    euler.push_back(x);
    nivel.push_back(niv);
    prima[x] = euler.size()-1;
    for(auto next: g[x]) {
        dfs(next, niv+1);
        euler.push_back(x);
        nivel.push_back(niv);
    }
}

void RMQ() {
    int k = euler.size();

    for(int i = 1; i<= k; i++)
        rmq[0][i] = i;
    for(int i = 2; i <= k; i++)
        lg[i] = lg[i/2]+1;

    for(int i = 1; (1<<i) <= k; i++)
        for(int j = 1; (1<<i)+j <= k; j++) {
            rmq[i][j] = rmq[i-1][j];
            if(nivel[rmq[i-1][j]] > nivel[rmq[i-1][j+(1<<(i-1))]])
                rmq[i][j] = rmq[i-1][j+(1<<(i-1))];

        }
}

int lca(int x, int y) {
    int a = min(prima[x], prima[y]);
    int b = max(prima[x], prima[y]);

    int dif = b-a+1;
    int log = lg[dif];

    int res = rmq[log][a];
    if(nivel[res] > nivel[rmq[log][b-(1<<log)+1]])
        res = rmq[log][b-(1<<log)+1];

    return euler[res];
}

void solve() {
    dfs(1, 1);
    RMQ();

    while(m--) {
        int x, y;
        fin >> x >> y;
        fout << lca(x, y) << '\n';
    }
}

int main() {
    citire();
    solve();
}