Contest 3

　　A：非常裸的dp。

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#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 P 1000000007
#define N 32
int a,b,c,d,f[N][N][N][N][];
void inc(int &x,int y){x+=y;if (x>=P) x-=P;}
int main()
{
    freopen("count.in","r",stdin);
    freopen("count.out","w",stdout);
    a=read(),b=read(),c=read(),d=read();
    f[][][][][]=f[][][][][]=f[][][][][]=f[][][][][]=;
    for (int i=;i<a+b+c+d;i++)
        for (int x=;x<=a;x++)
            for (int y=;y<=b;y++)
                for (int z=;z<=c;z++)
                {
                    int t=i-x-y-z;
                    if (t>d) continue;
                    for (int p=;p<;p++)
                        for (int q=;q<;q++)
                        if (p!=q)
                        {
                            if (q==&&x<a) inc(f[x+][y][z][t][q],f[x][y][z][t][p]);
                            if (q==&&y<b) inc(f[x][y+][z][t][q],f[x][y][z][t][p]);
                            if (q==&&z<c) inc(f[x][y][z+][t][q],f[x][y][z][t][p]);
                            if (q==&&t<d) inc(f[x][y][z][t+][q],f[x][y][z][t][p]);
                        }
                }
    cout<<((f[a][b][c][d][]+f[a][b][c][d][])%P+(f[a][b][c][d][]+f[a][b][c][d][])%P)%P;
    return ;
}

　　B：非常裸的组合。我又学傻了第一眼居然容斥还好拍了。

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#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 200010
#define P 1000000007
int n,m,k,ans=,fac[N],inv[N];
int C(int n,int m){return 1ll*fac[n]*inv[m]%P*inv[n-m]%P;}
int main()
{
    freopen("array.in","r",stdin);
    freopen("array.out","w",stdout);
    n=read(),m=read(),k=read();
    fac[]=;for (int i=;i<=n+m;i++) fac[i]=1ll*fac[i-]*i%P;
    inv[]=inv[]=;for (int i=;i<=n+m;i++) inv[i]=P-1ll*(P/i)*inv[P%i]%P;
    for (int i=;i<=n+m;i++) inv[i]=1ll*inv[i]*inv[i-]%P;
    if (m-k&) {cout<<;return ;}
    cout<<1ll*C(n,k)*C((m-k>>)+n-,n-)%P;
    return ;
}

　　C：非常裸的meet in the middle。

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#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 300010
#define ll long long
int n,b[];
ll a[][N],l,r,ans,m;
void dfs(int k,int n,int p,ll s)
{
    if (k>n) a[p][++a[p][]]=s;
    else
    {
        dfs(k+,n,p,s);
        dfs(k+,n,p,s+b[k]);
    }
}
ll calc(ll k)
{
    ll s=;int x=a[][];
    for (int i=;i<=a[][];i++)
    {
        while (x&&a[][x]+a[][i]>k) x--;
        s+=x;
    }
    return s;
}
int main()
{
    freopen("subset.in","r",stdin);
    freopen("subset.out","w",stdout);
    n=read();cin>>m;
    for (int i=;i<=n;i++) r+=(b[i]=read());
    dfs(,n>>,,);
    dfs((n>>)+,n,,);
    sort(a[]+,a[]+a[][]+);
    sort(a[]+,a[]+a[][]+);
    while (l<=r)
    {
        ll mid=l+r>>;
        if (calc(mid)>=m) ans=mid,r=mid-;
        else l=mid+;
    }
    cout<<ans;
    return ;
}

　　result：300 rank1