A - Don't be late

#include <bits/stdc++.h>
using namespace std;
int main() {
    int d, t, s; 
    cin >> d >> t >> s;
    cout << (t * s >= d ? "Yes" : "No") << "
";
}

B - Substring

#include <bits/stdc++.h>
using namespace std;
int main() {
    string s, t;
    cin >> s >> t;
    int ans = INT_MAX;
    for (int i = 0; i + t.size() <= s.size(); i++) {
        int change = 0;
        for (int j = 0; j < t.size(); j++)
            if (s[i + j] != t[j]) 
                ++change;
        ans = min(ans, change);
    }
    cout << ans << "
";
}

C - Sum of product of pairs

#include <bits/stdc++.h>
using namespace std;
constexpr int mod = 1e9 + 7;
int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++)
        cin >> a[i];
    vector<long long> pref(n);
    for (int i = 0; i < n; i++) {
        if (i == 0) pref[i] = a[i];
        else pref[i] = pref[i - 1] + a[i];
    }
    long long ans = 0;
    for (int i = 0; i < n; i++) {
        ans += a[i] * ((pref[n - 1] - pref[i]) % mod) % mod;
        ans %= mod;
    }
    cout << ans << "
";
}

D - Friends

原题hdu 1856。

#include <bits/stdc++.h>
using namespace std;
constexpr int N = 2e5 + 100;

int fa[N], son_num[N];

int Find(int x) {
    if (fa[x] == x) return fa[x];
    else return fa[x] = Find(fa[x]);
}

void Union(int x, int y) {
    x = Find(x);
    y = Find(y);
    if (x != y) {
        fa[y] = x;
        son_num[x] += son_num[y];
    }
}

void Init() {
    for (int i = 0; i < N; i++) {
        fa[i] = i;
        son_num[i] = 1;
    }
}

int main() {
    Init();
    int n, m;
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        int x, y;
        cin >> x >> y;
        Union(x, y);
    }
    cout << *max_element(son_num, son_num + N) << "
";
}

E - Coprime

#include <bits/stdc++.h>
using namespace std;
constexpr int N = 1e6 + 100;

int p[N];

void init() {
    for (int i = 2; i < N; i++) {
        if (p[i]) continue;
        for (int j = i; j < N; j += i) {
            p[j] = i;
        }
    }
}

int main() {
    init();
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++)
        cin >> a[i];
    vector<set<int>> div(n);
    for (int i = 0; i < n; i++) {
        for (int j = a[i]; p[j] != 0; j /= p[j]) {
            div[i].insert(p[j]);
        }
    }
    bool pairwise = true;
    map<int, int> cnt;
    for (int i = 0; i < n; i++) {
        for (const auto &x : div[i]) {
            if (++cnt[x] >= 2)
                pairwise = false;
        }
    }
    bool setwise = false;
    int gcd = 0;
    for (int i = 0; i < n; i++)
        gcd = __gcd(gcd, a[i]);
    if (gcd == 1) 
        setwise = true;
    if (pairwise)
        cout << "pairwise coprime" << "
";
    else if (setwise)
        cout << "setwise coprime" << "
";
    else
        cout << "not coprime" << "
";
}