Cod sursă (job #365514)

#include<fstream>
#include<algorithm>
#define DN 100005
#define x first
#define y second
using namespace std;
ifstream fin("infasuratoare.in");
ofstream fout("infasuratoare.out");
int n,top,st[DN];
typedef pair<float,float>pii;
pii a[DN];
int cmp(pii A,pii B)
{
    return (A.y<B.y||(A.y==B.y&&A.x<B.y));
}
double det(pii A,pii B,pii C)
{
    return A.x*B.y+B.x*C.y+C.x*A.y-B.y*C.x-C.y*A.x-A.y*B.x;
}
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>a[i].x>>a[i].y;
    sort(a+1,a+n+1,cmp);
    top=2;
    st[1]=1;
    st[2]=2;
    for(int i=3;i<=n;i++)
    {
        while(top>1&&det(a[i],a[st[top]],a[st[top-1]])<=0)
            top--;
        top++;
        st[top]=i;
    }
    for(int i=n-1;i>=1;i--)
    {
        while(top>1&&det(a[i],a[st[top]],a[st[top-1]])<=0)
            top--;
        top++;
        st[top]=i;
    }
    fout<<top-1<<'\n';
    for(int i=1;i<top;i++)
        fout<<a[st[i]].x<<' '<<a[st[i]].y<<'\n';
}