（一个月前的比赛）

2069A - Was there an Array?

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#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);

    int t;
    cin >> t;

    while (t--) {
        int n;
        cin >> n;
        n -= 2;

        vector<int> b(n);
        for (int i = 0; i < n; i++) {
            cin >> b[i];
        }

        bool flag = 1;
        for (int i = 1; i < n - 1; i++) {
            if (b[i - 1] == 1 && b[i] == 0 && b[i + 1] == 1) {
                flag = 0;
            }
        }
        cout << (flag ? "YES" : "NO") << endl;
    }
    
    return 0;
}

2069B - Set of Strangers

对于每种颜色，至多两次（按国际象棋棋盘黑白染色，只需要两次），如果有相邻，必然两次，否则一次。

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#include <bits/stdc++.h>
using namespace std;
using LL = long long;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);

    int t;
    cin >> t;

    while (t--) {
        int n, m;
        cin >> n >> m;

        vector a(n, vector<int>(m));
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                cin >> a[i][j];
                a[i][j]--;
            }
        }

        vector<int> cnt(n * m);
        vector<int> flag(n * m);
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                cnt[a[i][j]]++;
                if (i && a[i][j] == a[i - 1][j]) {
                    flag[a[i][j]] = 1;
                }
                if (j && a[i][j] == a[i][j - 1]) {
                    flag[a[i][j]] = 1;
                }
            }
        }

        for (int i = 0; i < n * m; i++) {
            cnt[i] = cnt[i] > 0;
            cnt[i] += flag[i];
        }

        cout << (accumulate(cnt.begin(), cnt.end(), 0LL) - *max_element(cnt.begin(), cnt.end())) << endl;
    }
    
    return 0;
}

2069C - Beautiful Sequence

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#include <bits/stdc++.h>
using namespace std;
using LL = long long;

constexpr LL mod = 998244353;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);

    int t;
    cin >> t;

    while (t--) {
        int n;
        cin >> n;

        vector<int> a(n);
        for (int i = 0; i < n; i++) {
            cin >> a[i];
        }

        LL res = 0;
        LL sum = 0;
        int cnt1 = 0;
        for (int i = 0; i < n; i++) {
            if (a[i] == 1) {
                cnt1++;
            } else if (a[i] == 2) {
                sum <<= 1;
                sum += cnt1;
                sum %= mod;
            } else {
                res += sum;
                res %= mod;
            }
        }
        cout << res << endl;
    }
    
    return 0;
}