c_cpp uva681

Posted 2021-05-14

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

template <typename T>
class _vertex
{
public:
    bool operator <(const _vertex& v) const
    {
        return (y != v.y) ? (y < v.y) : (x < v.x);
    }

    static T cross(const _vertex& o, const _vertex& a, const _vertex& b)
    {
        return (a.x - o.x)*(b.y - o.y) - (a.y - o.y)*(b.x - o.x);
    }

private:
    friend istream& operator >>(istream& is, _vertex& v)
    {
        is >> v.x >> v.y;
        return is;
    }

    friend ostream& operator <<(ostream& os, const _vertex& v)
    {
        os << v.x << " " << v.y;
        return os;
    }

private:
    T x, y;
};

using vertex = _vertex<int>;

vector<vertex> find_convex_hull(vector<vertex>&& contour)
{
    sort(contour.begin(), contour.end());

    if (contour.size() <= 3) return contour; // It should still be sorted to fulfill the required order.

    auto back = [](const vector<vertex>& container, size_t offset)
    {
        return *(container.rbegin() + offset);
    };

    vector<vertex> convex;

    for (auto it = contour.cbegin(); it != contour.cend(); it++)
    {
        while (convex.size() > 1 && vertex::cross(back(convex, 1), back(convex, 0), *it) <= 0) convex.pop_back();
        convex.push_back(*it);
    }

    size_t s = convex.size();
    for (auto it = contour.rbegin(); it != contour.rend(); it++)
    {
        while (convex.size() > s && vertex::cross(back(convex, 1), back(convex, 0), *it) <= 0) convex.pop_back();
        convex.push_back(*it);
    }

    return convex;
}

void test_case()
{
    size_t n;
    cin >> n;
    vector<vertex> contour(n);
    for (auto& v : contour) cin >> v;

    auto convex = find_convex_hull(move(contour));

    cout << convex.size() << endl;
    for (const auto& v : convex) cout << v << endl;
}

int main()
{
    size_t n;
    cin >> n;
    cout << n << endl;
    while (n--)
    {
        test_case();

        if (n > 0)
        {
            int s;
            cin >> s;
            cout << -1 << endl;
        }
    }
    return 0;
}