A. Alternating Sum of Numbers

#include<bits/stdc++.h>

using namespace std;

using i32 = int32_t;
using i64 = long long;
using i128 = __int128;

using vi = vector<int>;
using pii = pair<int, int>;

const i32 inf = INT_MAX / 2;
const i64 INF = LLONG_MAX / 2;

void solve() {
    int n, res = 0;
    cin >> n;
    for (int i = 1, x; i <= n; i++) {
        cin >> x;
        if (i & 1) res += x;
        else res -= x;
    }
    cout << res << "\n";
    return;
}


i32 main() {
    ios::sync_with_stdio(false), cin.tie(nullptr);
    int T;
    cin >> T;
    while (T--)
        solve();
    return 0;
}

B. Three Brothers

#include<bits/stdc++.h>

using namespace std;

using i32 = int32_t;
using i64 = long long;
using i128 = __int128;

//#define int i64

using vi = vector<int>;
using pii = pair<int, int>;

const i32 inf = INT_MAX / 2;
const i64 INF = LLONG_MAX / 2;


i32 main() {
    ios::sync_with_stdio(false), cin.tie(nullptr);
    int x , y;
    cin >> x >> y;
    cout << 6 - x - y;
    return 0;
}

C1. Message Transmission Error (easy version)

#include<bits/stdc++.h>

using namespace std;

using i32 = int32_t;
using i64 = long long;
using i128 = __int128;

//#define int i64

using vi = vector<int>;
using pii = pair<int, int>;

const i32 inf = INT_MAX / 2;
const i64 INF = LLONG_MAX / 2;


i32 main() {
    ios::sync_with_stdio(false), cin.tie(nullptr);
    string s;
    cin >> s;
    int n = s.size();
    for (int i = 1; i < n ; i++) {
        if( i * 2 <=n) continue;
        string t = s.substr(0, i);
        if (t == s.substr(n - i, i)) {
            cout << "YES\n" << t << "\n";
            return 0;
        }
    }
    cout << "NO\n";
    return 0;
}

C2. Message Transmission Error (hard version)

直接暴力做肯定不行了，这里可以用到kmp中的前缀和函数来计算

#include<bits/stdc++.h>

using namespace std;

using i32 = int32_t;
using i64 = long long;
using i128 = __int128;

//#define int i64

using vi = vector<int>;
using pii = pair<int, int>;

const i32 inf = INT_MAX / 2;
const i64 INF = LLONG_MAX / 2;

vi prefix_function(const string &s) {
    int n = s.size();
    vi pi(n);
    for (int i = 1, j; i < n; i++) {
        j = pi[i - 1];
        while (j > 0 and s[i] != s[j])
            j = pi[j - 1];
        if (s[i] == s[j]) j++;
        pi[i] = j;
    }
    return pi;
}

i32 main() {
    ios::sync_with_stdio(false), cin.tie(nullptr);
    string s;
    cin >> s;
    int n = s.size();
    vi pi = prefix_function(s);
    if(pi[n - 1] * 2 > n) {
        cout << "YES\n" << s.substr(0 , pi[n - 1]) << "\n";
    }else{
        cout << "NO\n";
    }
    return 0;
}