Codeforces_839

A.每天更新判断。

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

int n,k,a[105];

int main()
{
    ios::sync_with_stdio(0);
    cin >> n >> k;
    for(int i = 1;i <= n;i++)   cin >> a[i];
    int ans = 0,now = 0;
    for(int i = 1;i <= n;i++)
    {
        now += a[i];
        if(now > 8)
        {
            now -= 8;
            ans += 8;
        }
        else
        {
            ans += now;
            now = 0;
        }
        if(ans >= k)
        {
            cout << i << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}

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

int n,k,a[10005];

int main()
{
    ios::sync_with_stdio(0);
    cin >> n >> k;
    for(int i = 1;i <= k;i++)   cin >> a[i];
    int cnt4 = n;
    for(int i = 1;i <= k;i++)
    {
        while(a[i] >= 4 && cnt4)
        {
            a[i] -= 4;
            cnt4--;
        }
    }
    int cnt2 = cnt4+2*n;
    for(int i = 1;i <= k;i++)
    {
        while(a[i] >= 2 && cnt2)
        {
            a[i] -= 2;
            cnt2--;
        }
    }
    int cnt1 = cnt2+cnt4;
    for(int i = 1;i <= k;i++)   cnt1 -= a[i];
    if(cnt1 >= 0)   cout << "YES" << endl;
    else    cout << "NO" << endl;
    return 0;
}

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

int n;
vector<int> v[100005];

double dfs(int now,int pre)
{
    double ans = 1;
    int sz = v[now].size();
    if(now != 1)    sz--;
    for(int i = 0;i < v[now].size();i++)
    {
        int t = v[now][i];
        if(t == pre)    continue;
        ans += dfs(t,now)/sz;
    }
    return ans;
}

int main()
{
    ios::sync_with_stdio(0);
    cin >> n;
    for(int i = 1;i < n;i++)
    {
        int x,y;
        cin >> x >> y;
        v[x].push_back(y);
        v[y].push_back(x);
    }
    cout << fixed << setprecision(10) << dfs(1,0)-1 << endl;
    return 0;
}

但事实上，某些数因子会被比它的因子替代，我们按因子从大到小容斥。

#include<bits/stdc++.h>
#define MOD 1000000007
using namespace std;

int n;
long long two[200005],cnt[1000005] = {0},dp[1000005];

int main()
{
    ios::sync_with_stdio(0);
    two[0] = 1;
    for(int i = 1;i <= 200000;i++)  two[i] = two[i-1]*2%MOD;
    cin >> n;
    int maxx = 0;
    for(int i = 1;i <= n;i++)
    {
        int x;
        cin >> x;
        cnt[x]++;
        maxx = max(maxx,x);
    }
    long long ans = 0;
    for(int i = maxx;i > 1;i--)
    {
        long long t = 0;
        for(int j = i;j <= maxx;j += i) t += cnt[j];
        if(t == 0)  continue;
        dp[i] = t*two[t-1]%MOD;
        for(int j = 2*i;j <= maxx;j += i)   dp[i] = (dp[i]-dp[j]+MOD)%MOD;
        ans = (ans+dp[i]*i)%MOD;
    }
    cout << ans << endl;
    return 0;
}