越狱

盒子与球

斯特林第二类数（用dp求）*盒子的阶乘

int dp[11][11]; //n>=k
int A(int x){int res=1;fer(i,2,x+1)res*=i;return res;
}
signed main(){IOS;dp[2][1]=dp[2][2]=dp[1][1]=1;fer(i,3,11){dp[i][1]=1;fer(j,2,i){dp[i][j]=j*dp[i-1][j]+dp[i-1][j-1];}dp[i][i]=1;}int n,r;cin>>n>>r;cout<<dp[n][r]*A(r);return 0;
}

矩阵乘法

const int N=1e3+1,mod=1e9+7;
int a[N][N],b[N][N],res[N][N];
signed main(){IOS;int l,m,n; cin>>l>>m>>n;fer(i,0,l){fer(j,0,m)cin>>a[i][j];}fer(i,0,m){fer(j,0,n){cin>>b[i][j];}}//result l*nfer(i,0,l){fer(j,0,n){fer(k,0,m){res[i][j]+=a[i][k]*b[k][j];}}}fer(i,0,l){fer(j,0,n){cout<<res[i][j]<<" ";}cout<<endl;}return 0;
}

找朋友

最小的：1、1、1、1、……、n-m+1
最大的：n/m平均分，n%m平均分到每个小组里
res=$C_x^2$

int C(int x,int k){int res=1;for(int i=x;i>x-k;i--){res*=i;}fer(i,2,k+1)res/=i;return res;
}
signed main(){IOS;int n,m;cin>>n>>m;int res1=0,res2=0;int x=n/m;int y=n%m;//y个 C_{x+1}^2,(m-y)个C_x^2 res1=y*C(x+1,2)+(m-y)*C(x,2);res2=C(n-m+1,2);cout<<res1<<" "<<res2;return 0;
}

点餐

signed main(){IOS;int a;cin>>a;int b[3];fer(i,0,3)cin>>b[i];if(b[0]+b[1]+b[2]<=2*a)cout<<"YES";else cout<<"NO";return 0;
}