BZOJ 1037 生日聚会Party（DP）

int n,m,K,f[N][N][21][21];

void up(int &x,int y)
{
    x+=y;
    if(x>=mod) x-=mod;
}


int main()
{
    RD(n,m,K);
    int i,j,k,t;
    f[0][0][0][0]=1;
    FOR0(i,n+1) FOR0(j,m+1) FOR0(k,K+1) FOR0(t,K+1) if(f[i][j][k][t])
    {
        if(i<n&&k<K) up(f[i+1][j][k+1][max(t-1,0)],f[i][j][k][t]);
        if(j<m&&t<K) up(f[i][j+1][max(k-1,0)][t+1],f[i][j][k][t]);
    }
    int ans=0;
    FOR0(i,K+1) FOR0(j,K+1) up(ans,f[n][m][i][j]);
    PR(ans);
    return 0;
}