339D - Xenia and Bit Operations - CodeForces Solution

#include <iostream>
#include <vector>

using namespace std;

vector <long long> a;
vector <long long> t;

void build(int v, int l, int r) {
    if (r - l == 1) {
        t[v] = a[l];
        return;
    }
    int m = (l + r) / 2;
    build(2 * v, l, m);
    build(2 * v + 1, m, r);
    if (__lg(r - l) % 2 == 1) {
        t[v] = t[2 * v] | t[2 * v + 1];
    } else {
        t[v] = t[2 * v] ^ t[2 * v + 1];
    }
}


void upd(int v, int l, int r, int i, long long x) {
    if (r - l == 1) {
        t[v] = x;
        return;
    }
    int m = (l + r) / 2;
    if (i < m) {
        upd(2 * v, l, m, i, x);
    } else {
        upd(2 * v + 1, m, r, i, x);
    }
    if (__lg(r - l) % 2 == 1) {
        t[v] = t[2 * v] | t[2 * v + 1];
    } else {
        t[v] = t[2 * v] ^ t[2 * v + 1];
    }
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    a.resize(1 << n);
    t.resize(1 << (n + 2));
    for (int i = 0; i < (1 << n); ++i) {
        cin >> a[i];
    }
    build(1, 0, (1 << n));
    for (int i = 0; i < m; ++i) {
        int p, b;
        cin >> p >> b;
        upd(1, 0, (1 << n), p - 1, b);
        cout << t[1] << "\n";
    }
}

/* Tue Jan 31 2023 11:46:58 GMT+0300 (Moscow Standard Time) */