Codeforces_841

A.统计每个字母数量，比较是否超过k。

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

int n,k,cnt[26] = {0};
string s;

int main()
{
    ios::sync_with_stdio(0);
    cin >> n >> k >> s;
    for(int i = 0;i < s.length();i++)   cnt[s[i]-'a']++;
    for(int i = 0;i < 26;i++)
    {
        if(cnt[i] > k)
        {
            cout << "NO" << endl;
            return 0;
        }
    }
    cout << "YES" << endl;
    return 0;
}

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B.只要有奇数，First必赢，因为总能到奇数个数为奇数个的情况。

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

int n,a[1000005];

int main()
{
    ios::sync_with_stdio(0);
    cin >> n;
    int cnt = 0;
    for(int i = 1;i <= n;i++)
    {
        cin >> a[i];
        if(a[i]%2)  cnt++;
    }
    if(cnt > 0)   cout << "First" << endl;
    else    cout << "Second" << endl;
    return 0;
}

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C.当在[1,x]中随机取一个的时候，最小值期望为x/2，取的次数越多，期望越小。我们把优先次数少的分给大的数。

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

int n,a[200005],ans[200005];
struct xx
{
    int x,id;
    friend bool operator <(xx a,xx b)
    {
        return a.x > b.x;
    }
}b[200005];

int main()
{
    ios::sync_with_stdio(0);
    cin >> n;
    for(int i = 1;i <= n;i++)   cin >> a[i];
    for(int i = 1;i <= n;i++)
    {
        cin >> b[i].x;
        b[i].id = i;
    }
    sort(a+1,a+1+n);
    sort(b+1,b+1+n);
    for(int i = 1;i <= n;i++)   ans[b[i].id] = a[i];
    for(int i = 1;i <= n;i++)   cout << ans[i] << " ";
    cout << endl;
    return 0;
}

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D.首先图是连通的，若有-1的点存在，则必可以通过这个点，dfs把其它点都调整好。

若没有-1的点存在，我们随便选一个点，dfs下去，最后判断这个点是否符合。

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

int n,m,a[300005],vis[300005] = {0},dep[300005] = {0};
struct xxx
{
    int to,id;
    xxx(int a,int b):to(a),id(b){};
};
vector<xxx> v[300005];
vector<int> ans;

bool dfs(int now,int pre)
{
    vis[now] = 1;
    for(int i = 0;i < v[now].size();i++)
    {
        int t = v[now][i].to;
        if(t == pre || vis[t])    continue;
        if(dfs(t,now))
        {
            dep[now]++;
            ans.push_back(v[now][i].id);
        }
    }
    if(a[now] == 1 && dep[now]%2 == 0 || a[now] == 0 && dep[now]%2 == 1)    return 1;
    return 0;
}

int main()
{
    ios::sync_with_stdio(0);
    cin >> n >> m;
    for(int i = 1;i <= n;i++)   cin >> a[i];
    for(int i = 1;i <= m;i++)
    {
        int x,y;
        cin >> x >> y;
        v[x].push_back(xxx(y,i));
        v[y].push_back(xxx(x,i));
    }
    int ok = 0;
    for(int i = 1;i <= n;i++)
    {
        if(a[i] == -1)
        {
            dfs(i,0);
            ok = 1;
            break;
        }
    }
    if(!ok && dfs(1,0))
    {
        cout << -1 << endl;
        return 0;
    }
    cout << ans.size() << endl;
    for(int i = 0;i < ans.size();i++)   cout << ans[i] << " ";
    cout << endl;
    return 0;
}

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