Language: C++

#include "bits/stdc++.h"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld=double;
ld eps=1e-7;
/* ??????????????¬ */

#include <complex>

typedef complex<ld> Point;


const ld pi = acos(-1.0);
const ld dtop = pi / 180.;
const ld ptod = 1. / dtop;

namespace std {
	bool operator<(const Point &lhs, const Point &rhs) {
		if (lhs.real() < rhs.real() - eps) return true;
		if (lhs.real() > rhs.real() + eps) return false;
		return lhs.imag() < rhs.imag();
	}
}

// ????????\???
Point input_Point() {
	ld x, y;
	cin >> x >> y;
	return Point(x, y);
}

// ????????????????????????
bool eq(const ld a, const ld b) {
	return (abs(a - b) < eps);
}

// ??????
ld dot(const Point& a, const Point& b) {
	return real(conj(a) * b);
}

// ??????
ld cross(const Point& a, const Point& b) {
	return imag(conj(a) * b);
}

// ??´????????????
class Line {
public:
	Point a, b;
	Line() : a(Point(0, 0)), b(Point(0, 0)) {}
	Line(Point a, Point b) : a(a), b(b) {}
	Point operator[](const int _num)const {
		if (_num == 0)return a;
		else if (_num == 1)return b;
		else {
			assert(false);
			return Point();
		}
	}
};

// ????????????
class Circle {
public:
	Point p;
	ld r;
	Circle() : p(Point(0, 0)), r(0) {}
	Circle(Point p, ld r) : p(p), r(r) {}
};

// ccw
// 1: a,b,c??????????¨???¨?????????????????¶
//-1: a,b,c???????¨???¨?????????????????¶
// 2: c,a,b???????????´???????????¶
//-2: a,b,c???????????´???????????¶
// 0: a,c,b???????????´???????????¶
int ccw(const Point& a, const Point &b, const Point &c) {
	const Point nb(b - a);
	const Point nc(c - a);
	if (cross(nb, nc) > eps) return 1;   // a,b,c??????????¨???¨?????????????????¶
	if (cross(nb, nc) < -eps) return -1; // a,b,c???????¨???¨?????????????????¶
	if (dot(nb, nc) < 0) return 2;       // c,a,b???????????´???????????¶
	if (norm(nb) < norm(nc)) return -2;  // a,b,c???????????´???????????¶
	return 0;                          // a,c,b???????????´???????????¶
}


/* ???????????? */

// ??´?????¨??´??????????????????
bool isis_ll(const Line& l, const Line& m) {
	return !eq(cross(l.b - l.a, m.b - m.a), 0);
}

// ??´?????¨?????????????????????
bool isis_ls(const Line& l, const Line& s) {
	return isis_ll(l, s) &&
		(cross(l.b - l.a, s.a - l.a) * cross(l.b - l.a, s.b - l.a) < eps);
}

// ????????¨?????????????????????
bool isis_ss(const Line& s, const Line& t) {
	return ccw(s.a, s.b, t.a) * ccw(s.a, s.b, t.b) <= 0 &&
		ccw(t.a, t.b, s.a) * ccw(t.a, t.b, s.b) <= 0;
}

// ????????´????????????
bool isis_lp(const Line& l, const Point& p) {
	return (abs(cross(l.b - p, l.a - p)) < eps);
}

// ?????????????????????
bool isis_sp(const Line& s, const Point& p) {
	return (abs(s.a - p) + abs(s.b - p) - abs(s.b - s.a) < eps);
}

// ??????????¶?
Point proj(const Line &l, const Point& p) {
	ld t = dot(p - l.a, l.b - l.a) / norm(l.a - l.b);
	return l.a + t * (l.b - l.a);
}

//???????±??????????????????????
Point reflect(const Line &l, const Point &p) {
	Point pr = proj(l, p);
	return pr * 2.l - p;
}

// ??´?????¨??´????????????
Point is_ll(const Line &s, const Line& t) {
	Point sv = s.b - s.a, tv = t.b - t.a;
	assert(cross(sv, tv) != 0);
	return s.a + sv * cross(tv, t.a - s.a) / cross(tv, sv);
}
// ??´?????¨??´????????????
vector<Point> is_ll2(const Line &s, const Line& t) {
	Point sv = s.b - s.a, tv = t.b - t.a;
	if (cross(sv, tv) != 0)return vector<Point>(1, is_ll(s, t));
	else {
		vector<Point>ans;
		for (int k = 0; k < 2; ++k) {
			if (isis_sp(s, t[k]) && find(ans.begin(), ans.end(), t[k]) == ans.end())ans.push_back(t[k]);
			if (isis_sp(t, s[k]) && find(ans.begin(), ans.end(), s[k]) == ans.end())ans.push_back(s[k]);
		}
		return ans;
	}
}
// ????????¨???????????????
//???????????£????????¨???????????¨assert(false)
Point is_ss(const Line &s, const Line& t) {
	if (isis_ss(s, t)) {
		for (int k = 0; k < 2; ++k) {
			for (int l = 0; l < 2; ++l) {
				if (s[k] == t[l])return s[k];
			}
		}
		return is_ll(s, t);
	}
	else {
		//??????isis_ss?????????
		assert(false);
		return Point(0, 0);
	}
}
// ????????¨???????????????
vector<Point> is_ss2(const Line &s, const Line& t) {
	vector<Point> kouho(is_ll2(s, t));
	vector<Point>ans;
	for (auto p : kouho) {
		if (isis_sp(s, p) && isis_sp(t, p))ans.emplace_back(p);
	}
	return ans;
}
// ??´?????¨???????????¢
ld dist_lp(const Line& l, const Point& p) {
	return abs(p - proj(l, p));
}

//??´?????¨??´???????????¢
ld dist_ll(const Line& l, const Line& m) {
	return isis_ll(l, m) ? 0 : dist_lp(l, m.a);
}

// ??´?????¨??????????????¢
ld dist_ls(const Line& l, const Line& s) {
	return isis_ls(l, s) ? 0 : min(dist_lp(l, s.a), dist_lp(l, s.b));
}

// ????????¨???????????¢
ld dist_sp(const Line& s, const Point& p) {
	Point r = proj(s, p);
	return isis_sp(s, r) ? abs(r - p) : min(abs(s.a - p), abs(s.b - p));
}

// ????????¨??????????????¢
ld dist_ss(const Line& s, const Line& t) {
	if (isis_ss(s, t)) return 0;
	return min({ dist_sp(s, t.a), dist_sp(s, t.b), dist_sp(t, s.a), dist_sp(t, s.b) });
}

int main()
{
	ld ans=1e18;
	int N;cin>>N;
	vector<Line>ls;
	map<pair<int,int>,int>mp;
	for (int i = 0; i < N; ++i) {
		int a,b,c,d;cin>>a>>b>>c>>d;
		ls.push_back(Line(Point(a,b),Point(c,d)));
		mp[make_pair(a,b)]++;
		mp[make_pair(c,d)]++;
	}
	bool flag=false;
	for (auto m : mp) {
		if(m.second>=2)flag=true;
	}
	if (flag) {
		ans=0;
	}
	else {
		for (int i = 0; i < N; ++i) {
			for (int j =i+1; j < N; ++j) {
				
				ans=min(ans,dist_ss(ls[i],ls[j]));
				
			}
		}
	}
	cout<<setprecision(10)<<fixed<<ans<<endl;
	
	return 0;
}

Compiler output:
a.cc: In function 'Point reflect(const Line&, const Point&)':
a.cc:129:19: error: no match for 'operator*' (operand types are 'Point' {aka 'std::complex<double>'} and 'long double')
  129 |         return pr * 2.l - p;
      |                ~~ ^ ~~~
      |                |    |
      |                |    long double
      |                Point {aka std::complex<double>}
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127,
                 from a.cc:1:
/usr/include/c++/14/complex:400:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const complex<_Tp>&, const complex<_Tp>&)'
  400 |     operator*(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:400:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   mismatched types 'const std::complex<_Tp>' and 'long double'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/complex:409:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const complex<_Tp>&, const _Tp&)'
  409 |     operator*(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:409:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   deduced conflicting types for parameter 'const _Tp' ('double' and 'long double')
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/complex:418:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const _Tp&, const complex<_Tp>&)'
  418 |     operator*(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:418:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   mismatched types 'const std::complex<_Tp>' and 'long double'
  129 |         return pr * 2.l - p;
      |                     ^~~
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   'Point' {aka 'std::complex<double>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   'Point' {aka 'std::complex<double>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'long double'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   'Point' {aka 'std::complex<double>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'long double'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const valarray<_Tp>&, const valarray<_Tp>&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   'Point' {aka 'std::complex<double>'} is not derived from 'const std::valarray<_Tp>'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   'Point' {aka 'std::complex<double>'} is not derived from 'const std::valarray<_Tp>'
  129 |         return pr * 2.l - p;
      |                     ^~~
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:129:21: note:   mismatched types 'const std::valarray<_Tp>' and 'long double'
  129 |         return pr * 2.l - p;
      |                     ^~~