So I am solving the prob: Find XOR sum of all subsets, where $$$n \lt 1000,\sum a_i\leq 2000000$$$, so I use bitset<2000001> to optimize it. Then the weird thing happened, I passed the prob with this code:

int main() {
    ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
    int n; cin >> n;

    bitset<2000001> dp;
    dp[0] = 1;
    for (int i = 0; i < n; i++) {
        int x; cin >> x;
        dp = dp ^ (dp << x);
    }

    int ans = 0;
    for (int j = 0; j <= 2000000; j++)
        if (dp[j])
            ans ^= j;

    cout << ans << '\n';

    return 0;
}

But, with this code:

----- same ----
    vector<int> w(n);
    bitset<2000001> dp;
    dp[0] = 1;
    for (int i = 0; i < n; i++) {
        cin >> w[i];
        dp = dp ^ (dp << w[i]);
    }
----- same ----

I got WA on some test cases, where my output is always 0. Is there any difference between these two approach? Why it failed with the second code?