Pick-up sticks · znzryb
## 思路讲解
> 可以假设顶层木棍不超过1000根。
这句话比较重要。于是我们选择去维护这个顶层木棍，不断用上面的木棍去看这些木棍是不是顶层木棍。
核心代码长这样。
```cpp
FOR(i,0,N-1){
Line a=A[i];
for(auto it=ans.begin();it!=ans.end();){
Line t=A[*it];
auto nit=next(it);
if(isseginterline(a,t) && isseginterline(t,a)){
ans.erase(it);
}
it=nit;
}
ans.EB(i);
}
```
## AC代码
[https://vjudge.net/solution/63275995](https://vjudge.net/solution/63275995)
::: 源代码
```cpp
// by Gospel_rock
#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define PB push_back
#define EB emplace_back
#define EM emplace
#define FI first
#define SE second
#define pct __builtin_popcountll
#define ctz __builtin_ctzll
#define SZ(a) ((long long) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define cend cerr<<"\n-----------\n"
#define fsp(x) fixed<<setprecision(x)
#define minn(vec) *min_element(vec.begin(),vec.end())
#define maxx(vec) *max_element(vec.begin(),vec.end())
using namespace std;
#ifdef LOCAL
template<typename T> void _print(T x) { cerr << x; }
// 可以打印多层数组，但是格式不是很好
template<typename T> void _print(vector<T> v) { cerr << "[ "; for (T i : v) _print(i), cerr << " "; cerr << "]"; }
template<typename T> void _print(set<T> s) { cerr << "{ "; for (T i : s) _print(i), cerr << " "; cerr << "}"; }
template<typename T, typename U> void _print(pair<T, U> p) { cerr << "{"; _print(p.first); cerr << ", "; _print(p.second); cerr << "}"; }
template<typename T, typename U> void _print(map<T, U> m) { cerr << "{ "; for (auto &p : m) _print(p), cerr << " "; cerr << "}"; }
// 打印两层数组，格式好一些
template<typename T> void _print(vector<vector<T>> v){cerr<<"{\n";for (auto i : v) _print(i), cerr << "\n";cerr<<" }";}
#endif
using ll = long long;using ull = unsigned long long;
using ld=double;using LD=long double;
using i128 = __int128;
using pdd = pair<ld,ld>;using plb = pair<ll,bool>;
using pll = pair<ll,ll>;using vll=vector<ll>;using vb=vector<bool>;
using tll = tuple<ll,ll,ll>;using vii=vector<int>;
using vpll = vector<pll>;using vtll = vector<tll>;
using vdb = vector<ld>;using vc=vector<char>;
using arr3 = array<ll,3> ;using arr2 = array<ll,2>;
using CD=complex<double>;
static constexpr ll MAXN=(ll)1e6+10,INF=(ll)1e17+3; // 1e17+3是素数 1e18+9是素数
static constexpr ll mod=(ll)1e9+7; // 998244353;
static constexpr double eps=1e-8;const double pi=acos(-1.0);
ll N,M,Q,X,K,T,lT;
inline void __();
signed main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
// cin>>lT;
while(cin>>N)
__();
return 0;
}
/*
*/
using Real = ld;
using Point = complex<Real>;
#define x() real()
#define y() imag()
Real cross(const Point &a, const Point &b) {
return (conj(a) * b).imag();
}
Real dot(const Point &a, const Point &b) {
return (conj(a) * b).real();
}
int sgn(Real x) {
if (-eps < x && x < eps) return 0;
return x < 0 ? -1 : 1;
}
namespace std
{
bool operator <(const Point& a, const Point& b){
return sgn(real(a) - real(b)) ? real(a) < real(b) : imag(a) <
imag(b);
}
bool operator ==(const Point& a, const Point& b){
if(!sgn(a.imag()-b.imag()) && !sgn(a.real()-b.real())){
return true;
}
return false;
}
bool operator !=(const Point& a, const Point& b){
if(!sgn(a.imag()-b.imag()) && !sgn(a.real()-b.real())){
return false;
}
return true;
}
istream& operator>>(istream& in, Point& c) {
Real r, i;
in >> r >> i;
c = Point(r, i);
return in;
}
}
// 采用两点式
struct Line{
Point a,b;
Line(){}
Line(Point a,Point b):a(a),b(b) {}
bool operator<(const Line&other) const{
return a!=other.a?a<other.a:b<other.b;
}
bool operator==(const Line&other) const{
return a==other.a && b==other.b;
}
};
istream& operator>>(istream& in, Line& line) {
Real a,b,c,d;
in >> a >> b >> c >> d;
line = Line(Point(a,b),Point(c,d));
return in;
}
// p+tv=q+sw
// (p-q) x w+tv x w = 0
// t=-(p-q)x w/(vxw)
// 传入的是方向式 拿到的是两直线交点
Point getintersect(const Line &a,const Line &b){
Point p=a.a, v=a.b-a.a,q=b.a,w=b.b-b.a;
Point u=p-q;
ld t=cross(w,u)/cross(v,w);
return p+v*t;
}
bool isseginterline(Line seg,Line line){
if(sgn(cross(line.a-seg.a, line.b-seg.a))*sgn(cross(line.a-seg.b, line.b-seg.b))>0){
return false;
}
return true;
}
inline void __(){
if(N==0) return;
vector<Line> A;
FOR(i,1,N){
Line a;
cin>>a;
A.EB(a);
}
vb vis(N+5);
list<int> ans;
cout<<"Top sticks: ";
// ROF(i,SZ(ans)-1,0){
// if(i!=0)cout<<ans[i]<<", ";
// else cout<<ans[i]<<".\n";
// }
FOR(i,0,N-1){
Line a=A[i];
for(auto it=ans.begin();it!=ans.end();){
Line t=A[*it];
auto nit=next(it);
if(isseginterline(a,t) && isseginterline(t,a)){
ans.erase(it);
}
it=nit;
}
ans.EB(i);
}
for(auto it=ans.begin();it!=ans.end();++it){
if(it!=(--ans.end())){
cout<< *it+1<<", ";
}else{
cout<< *it+1<<".\n";
}
}
}
/*
AC
https://vjudge.net/solution/63275995
*/
```
:::
## 心路历程（WA，TLE，MLE……）