A：签到。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
int x=0,f=1;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
return x*f;
}
int main()
{
int n=read(),m=read();
if (m*2-1<=n) cout<<"YES";else cout<<"NO";
return 0;
}
B：直接按欧拉路判，才不管只有四个点。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
int x=0,f=1;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
return x*f;
}
int a[5],fa[5];
int find(int x){return fa[x]==x?x:fa[x]=find(fa[x]);}
int main()
{
for (int i=1;i<=4;i++) fa[i]=i;
for (int i=1;i<=3;i++)
{
int x=read(),y=read();
a[x]++,a[y]++;fa[find(x)]=find(y);
}
for (int i=1;i<=4;i++)
if (find(i)!=find(1)) {cout<<"NO";return 0;}
int cnt=0;
for (int i=1;i<=4;i++)
{
if (a[i]&1) cnt++;
}
if (cnt==2) cout<<"YES";
else {cout<<"NO";return 0;}
return 0;
}
C：相当于可以用1代价获得1收益，用2代价获得b-a收益。瞎讨论即可。注意2代价获得b-a收益的前提是当前有至少a块饼干。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
int x=0,f=1;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
return x*f;
}
int main()
{
int n=read(),a=read(),b=read();
b-=a;
if (b<=2) cout<<n+1;
else
{
if (n<a) cout<<n+1;
else
{
n-=a-1;
ll ans=a;
ans+=1ll*b*(n/2);
if (n&1) ans++;
cout<<ans;
}
}
return 0;
}
D：相当于找一个形如0 非零偶数 奇数 非零偶数 0 的序列（每一段长度任意且可以为空），使该序列与原序列差的绝对值之和最小。赛时智商急剧下降搞了一大堆前缀和，最后屯完题交的时候网又卡了，发现wa掉的时候只剩下5min，于是就没救了。实际上直接dp不能再好写，即f[i][0/1/2/3/4]表示第i个位置在第j段中时的最小和。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
#define N 200010
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
int x=0,f=1;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
return x*f;
}
int n,a[N],delta[N];
ll f[N][5];
//ll ans,f[N][2],s[N],g[N][2],h[N];
int main()
{
n=read();
for (int i=1;i<=n;i++) a[i]=read();
/*for (int i=1;i<=n;i++)
if (a[i]==0) delta[i]=2;
else delta[i]=a[i]&1;
for (int i=1;i<=n;i++) g[i][0]=g[i-1][0]+delta[i],s[i]=s[i-1]+a[i];
ll u=0;
for (int i=1;i<=n;i++)
{
u=min(u,s[i]-g[i][0]);
f[i][0]=g[i][0]+u;
}
for (int i=n;i>=1;i--) g[i][1]=g[i+1][1]+delta[i],s[i]=s[i+1]+a[i];
u=0;
for (int i=n;i>=1;i--)
{
u=min(u,s[i]-g[i][1]);
f[i][1]=g[i][1]+u;
}
for (int i=1;i<=n;i++) ans+=a[i];
u=0;
for (int i=1;i<=n;i++) ans=min(ans,f[i][1]+s[i-1]);
for (int i=1;i<=n;i++) h[i]=h[i-1]+(a[i]&1^1);
for (int i=1;i<=n+1;i++)
{
ans=min(ans,f[i][1]+u+h[i-1]);
u=min(u,f[i][0]-h[i]);
}*/
for (int i=1;i<=n;i++)
{
f[i][0]=f[i-1][0];
for (int j=1;j<=4;j++)
f[i][j]=min(f[i][j-1],f[i-1][j]);
f[i][0]+=a[i],f[i][4]+=a[i];
f[i][1]+=a[i]==0?2:(a[i]&1);
f[i][3]+=a[i]==0?2:(a[i]&1);
f[i][2]+=a[i]&1^1;
}
ll ans=f[n][0];for (int i=1;i<=4;i++) ans=min(ans,f[n][i]);
cout<<ans;
return 0;
}
F：大胆猜想序列合法当且仅当序列前i个位置的红球个数<=前i个人的红球个数，序列前i个位置的蓝球个数<=前i个人的蓝球个数，然后就是个思博dp了。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
#define N 2010
#define P 998244353
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
int x=0,f=1;char c=getchar();
while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
return x*f;
}
int n,a[N],u,v,s[N][2],f[N<<1][N<<1];
char S[N];
void inc(int &x,int y){x+=y;if (x>=P) x-=P;}
int main()
{
scanf("%s",S+1);
n=strlen(S+1);
for (int i=1;i<=n;i++) a[i]=S[i]-'0';
for (int i=1;i<=n;i++) u+=2-a[i],v+=a[i];
for (int i=1;i<=n;i++) s[i][0]=s[i-1][0]+2-a[i],s[i][1]=s[i-1][1]+a[i];
f[0][0]=1;
for (int i=0;i<=u;i++)
for (int j=0;j<=v;j++)
if (i|j)
{
if (s[min(n,i+j)][0]>=i&&s[min(n,i+j)][1]>=j)
{
if (i) inc(f[i][j],f[i-1][j]);
if (j) inc(f[i][j],f[i][j-1]);
}
}
cout<<f[u][v];
return 0;
}
result：rank 207 rating +38 莫名其妙上黄了但非常不爽啊。