[18005] Even or Odd?

https://www.acmicpc.net/problem/18005

18005번: Even or Odd?

1) 1부터 n까지 더한 경우

2) 2부터 n+1까지 더한 경우

위 2가지만 체크하면 된다. 

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#include <bits/stdc++.h>

using namespace std;

#ifdef ONLINE_JUDGE

constexpr bool local = false;

#else

constexpr bool local = true;

#endif

using ll = long long;

using pi = pair<ll, ll>;

int main(void) {

    if (!local) ios_base::sync_with_stdio(0), cin.tie(0);

    ll n;

    cin >> n;

    ll sum = (n * (n + 1)) / 2LL;

    ll a = sum, b = sum + n;

    if (a & 1 and b & 1)

        cout << 1;

    else if ((a ^ b) & 1)

        cout << 0;

    else

        cout << 2;

    return 0;

}

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