Hi everyone,

I've recently started learning segment trees and was practicing with the problem: 339D — Xenia and Bit Operations.

I wrote the following C++ code to solve it. When I run the code locally using the CPH extension in VS Code, it works perfectly and gives the expected output for all test cases, including the one where it fails on Codeforces. However, when I submit the exact same code on Codeforces, it gives Wrong Answer on some test cases.

#include <bits/stdc++.h>
#include <ctype.h>
using namespace std;
#define int long long int
#define faster ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)

class Tree {
public:
    vector<int> seg;
    int lvl;
    string type;

    Tree(int n, int x) {
        seg.resize(4 * n + 1);
        if (x % 2) type = "or";
        else type = "xor";
    }

    void build(int ind, int lo, int hi, int arr[]) {
        if (lo == hi) {
            seg[ind] = arr[lo];
            return;
        }
        int mid = (lo + hi) >> 1;
        lvl++;
        build(2 * ind + 1, lo, mid, arr);
        build(2 * ind + 2, mid + 1, hi, arr);
        lvl--;

        if (type == "or") {
            if (lvl % 2) seg[ind] = seg[2 * ind + 1] ^ seg[2 * ind + 2];
            else seg[ind] = seg[2 * ind + 1] | seg[2 * ind + 2];
        } else {
            if (lvl % 2 == 0) seg[ind] = seg[2 * ind + 1] ^ seg[2 * ind + 2];
            else seg[ind] = seg[2 * ind + 1] | seg[2 * ind + 2];
        }
    }

    void update(int ind, int lo, int hi, int i, int val) {
        if (lo == hi) {
            seg[ind] = val;
            return;
        }
        int mid = (lo + hi) >> 1;
        lvl++;
        if (i <= mid) update(2 * ind + 1, lo, mid, i, val);
        else update(2 * ind + 2, mid + 1, hi, i, val);
        lvl--;

        if (type == "or") {
            if (lvl % 2) seg[ind] = seg[2 * ind + 1] ^ seg[2 * ind + 2];
            else seg[ind] = seg[2 * ind + 1] | seg[2 * ind + 2];
        } else {
            if (lvl % 2 == 0) seg[ind] = seg[2 * ind + 1] ^ seg[2 * ind + 2];
            else seg[ind] = seg[2 * ind + 1] | seg[2 * ind + 2];
        }
    }
};

void solve() {
    int n, q;
    cin >> n >> q;
    int x = n;
    n = 1 << n;
    int arr[n];
    for (int i = 0; i < n; i++) cin >> arr[i];
    Tree tree(n, x);
    tree.build(0, 0, n - 1, arr);

    while (q--) {
        int i, val;
        cin >> i >> val;
        i--;
        tree.lvl = 0;
        tree.update(0, 0, n - 1, i, val);
        cout << tree.seg[0] << endl;
    }
}

int32_t main() {
    solve();
    return 0;
}

I would be really grateful if someone could help me understand:

Is the problem with my code, or is it something related to compiler behavior on Codeforces?

Why does the same code behave correctly locally but gives WA on Codeforces?

Any suggestions or corrections are welcome. Thank you!