193 lines
3.5 KiB
C++
193 lines
3.5 KiB
C++
/* Problem URL: https://cses.fi/problemset/task/2195 */
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#include <bits/stdc++.h>
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#include <ext/pb_ds/assoc_container.hpp>
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#include <ext/pb_ds/tree_policy.hpp>
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using namespace std;
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using namespace __gnu_pbds;
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template <class T, class comp = less<>>
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using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;
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#define V vector
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#define rmin(a, b) a = min(a, b)
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#define rmax(a, b) a = max(a, b)
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#define rep(i, lim) for (int i = 0; i < (lim); i++)
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#define nrep(i, s, lim) for (int i = s; i < (lim); i++)
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#define repv(i, v) for (auto &i : (v))
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#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
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#define sortv(v) sort(v.begin(), v.end())
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#define all(v) (v).begin(), (v).end()
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using vi = vector<int>;
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using vvi = vector<vi>;
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using vvvi = vector<vvi>;
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using vvvvi = vector<vvvi>;
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using ll = long long;
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using vl = vector<ll>;
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using vvl = vector<vl>;
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using vvvl = vector<vvl>;
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using vvvvl = vector<vvvl>;
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template<class v>
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auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
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os << vec[0];
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for (size_t i = 1; i < vec.size(); i++) {
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os << ' ' << vec[i];
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}
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os << '\n';
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return os;
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}
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template<class v>
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auto operator>>(istream &is, vector<v> &vec)->istream& {
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for (auto &i : vec) {
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is >> i;
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}
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return is;
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}
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template<class v>
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auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
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for (auto &i : vec) {
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os << i[0];
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for (size_t j = 1; j < i.size(); j++) {
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os << ' ' << i[j];
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}
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os << '\n';
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}
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return os;
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}
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template<class v>
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auto operator>>(istream &is, vector<vector<v>> &vec)->istream& {
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for (auto &i : vec) {
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for (auto &j : i) {
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is >> j;
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}
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}
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return is;
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}
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const int oo = INT32_MAX >> 1;
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const ll OO = INT64_MAX >> 1;
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struct pt {
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ll x, y;
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pt() = default;
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pt(ll x, ll y): x(x), y(y) {}
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pt(const pt &a): x(a.x), y(a.y) {}
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friend istream &operator >> (istream &is, pt &a) {
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is >> a.x >> a.y;
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return is;
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}
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friend ostream &operator << (ostream &os, pt a) {
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os << a.x << ' ' << a.y << '\n';
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return os;
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}
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ll operator ^ (pt b) {
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return x * b.y - y * b.x;
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}
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pt operator - (pt b) {
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return pt(x - b.x, y - b.y);
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}
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bool operator < (pt b) {
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if (x == b.x) {
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return y < b.y;
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}
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return x < b.x;
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}
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};
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ll sarea2(pt a, pt b, pt c) {
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return (b - a) ^ (c - b);
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}
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int ccw(pt a, pt b, pt c)
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{
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ll ar = sarea2(a, b, c);
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return (ar > 0) - (ar < 0);
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}
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V<pt> convex_hull(V<pt> v)
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{
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sortv(v);
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if (v.size() <= 2) {
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return v;
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}
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int n = v.size();
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V<pt> ac;
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V<pt> tmp;
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rep(i, n) {
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while (ac.size() >= 2 && ccw(ac.end()[-2], ac.end()[-1], v[i]) > 0) {
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ac.pop_back();
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}
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ac.emplace_back(v[i]);
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}
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for (int i = n - 1; i >= 0; i--) {
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while (tmp.size() >= 2 && ccw(tmp.end()[-2], tmp.end()[-1], v[i]) > 0) {
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tmp.pop_back();
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}
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tmp.emplace_back(v[i]);
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}
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ac.pop_back();
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tmp.pop_back();
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repv(i, tmp) {
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ac.emplace_back(i);
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}
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return ac;
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}
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void pre()
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{
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}
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#define TEST 0
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void solve()
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{
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int n;
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cin >> n;
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V<pt> a(n);
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cin >> a;
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auto conv = convex_hull(a);
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cout << conv.size() << '\n';
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repv(i, conv) {
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cout << i.x << ' ' << i.y << '\n';
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}
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}
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int main()
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{
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ios::sync_with_stdio(false);
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cin.tie(nullptr);
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pre();
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int t;
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(TEST && cin >> t) || (t = 1);
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while (t--) {
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solve();
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}
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}
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