Codeforces Round #518 Div. 1没翻车记

　　A：设f[i][j][0/1]为前i个数第i位为j且第i位未满足/已满足限制的方案数。大力dp前缀和优化即可。

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
int read()
{
    int x=,f=;char c=getchar();
    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}
    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();
    return x*f;
}
#define N 100010
#define P 998244353
#define K 200
int n,a[N],f[N][K+][],g[N][K+][];//0 there is limit
void inc(int &x,int y){x+=y;if (x>=P) x-=P;}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("a.in","r",stdin);
    freopen("a.out","w",stdout);
#endif
    n=read();
    for (int i=;i<=n;i++) a[i]=read();
    f[][][]=;
    for (int i=;i<=K;i++) g[][i][]=;
    for (int i=;i<=n;i++)
    {
    if (a[i]!=-)
    {
        inc(f[i][a[i]][],(g[i-][K][]-g[i-][a[i]-][]+P)%P);
        inc(f[i][a[i]][],f[i-][a[i]][]);
        inc(f[i][a[i]][],g[i-][a[i]-][]),inc(f[i][a[i]][],g[i-][a[i]-][]);
    }
    else
    {
        for (int j=;j<=K;j++)
        {
            inc(f[i][j][],(g[i-][K][]-g[i-][j-][]+P)%P);
            inc(f[i][j][],f[i-][j][]);
            inc(f[i][j][],g[i-][j-][]),inc(f[i][j][],g[i-][j-][]);
        }
    }
    for (int j=;j<=K;j++)
    g[i][j][]=(f[i][j][]+g[i][j-][])%P,g[i][j][]=(f[i][j][]+g[i][j-][])%P;
    }
    if (a[n]!=-) cout<<f[n][a[n]][];
    else
    {
        int ans=;
        for (int i=;i<=;i++) inc(ans,f[n][i][]);
        cout<<ans;
    }
    return ;
}

　　B：显然若合法根一定是直径中点。跑几遍dfs。

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
int read()
{
    int x=,f=;char c=getchar();
    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}
    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();
    return x*f;
}
#define N 100010
int n,m,p[N],deep[N],fa[N],t=,root;
struct data{int to,nxt;
}edge[N<<];
void addedge(int x,int y){t++;edge[t].to=y,edge[t].nxt=p[x],p[x]=t;}
void dfs(int k)
{
    for (int i=p[k];i;i=edge[i].nxt)
    if (edge[i].to!=fa[k])
    {
        deep[edge[i].to]=deep[k]+;
        fa[edge[i].to]=k;
        dfs(edge[i].to);
    }
}
bool check(int k)
{
    bool flag=;
    int x=;
    for (int i=p[k];i;i=edge[i].nxt)
    if (edge[i].to!=fa[k]) x++;
    if (x==) return deep[k]==m;
    if (x>=)
    {
        for (int i=p[k];i;i=edge[i].nxt)
        if (edge[i].to!=fa[k]) flag=flag&&check(edge[i].to);
    }
    else flag=;
    return flag;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("b.in","r",stdin);
    freopen("b.out","w",stdout);
#endif
    n=read(),m=read();
    for (int i=;i<n;i++)
    {
        int x=read(),y=read();
        addedge(x,y),addedge(y,x);
    }
    dfs();
    for (int i=;i<=n;i++) if (deep[i]>=deep[root]) root=i;
    memset(deep,,sizeof(deep));
    memset(fa,,sizeof(fa));
    dfs(root);
    for (int i=;i<=n;i++) if (deep[i]>=deep[root]) root=i;
    if (deep[root]!=m*) {cout<<"No";return ;}
    for (int i=;i<=m;i++) root=fa[root];
    memset(deep,,sizeof(deep));
    memset(fa,,sizeof(fa));
    dfs(root);
    if (check(root)) cout<<"Yes";
    else cout<<"No";
    return ;
}

　　一题20min感觉非常友好。然后一看C是构造就弃了。D一点不会。E看不懂。hack不动。一个多小时无所事事地看着排名往下掉。

　　难得手速快了一次？

　　result：rank 95 rating +83 突然就上黄了感觉下一场就掉回去了啊。