Language: C++

#include <iostream>
#include <set>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

using ll = long long;

#define int ll

const int infll = 1e18 + 100;

bool cmp(pair<int, int>a, pair<int, int>b) {
	return a.second < b.second || (a.second == b.second && a.first < b.first);
}

signed main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n;
	cin >> n;
	vector<pair<int, int>> segments;
	for (int i = 0; i < n; i++) {
		int l, r;
		cin >> l >> r;
		segments.emplace_back(l, r);
	}
	int ans = 0;
	sort(segments.begin(), segments.end(), cmp);
	vector<pair<int, int>> suffix(n, { inf,-inf });
	vector<pair<int, int>> prefix(n, { inf, -inf });
	suffix.back() = segments.back();
	for (int i = n - 2; i >= 0; i--) {
		int l = suffix[i + 1].first;
		int r = suffix[i + 1].second;
		l = max(l, segments[i].first);
		r = min(r, segments[i].second);
		if (l > r) {
			break;
		}
		suffix[i] = { l,r };
	}
	prefix[0] = segments[0];
	for (int i = 1; i < n; i++) {
		int l = prefix[i - 1].first;
		int r = prefix[i - 1].second;
		l = max(l, segments[i].first);
		r = min(r, segments[i].second);
		if (l > r) {
			break;
		}
		prefix[i] = { l,r };
	}

	for (int i = n - 1; i > 0; i--) {
		int l1 = suffix[i].first;
		int r1 = suffix[i].second;
		int l2 = prefix[i - 1].first;
		int r2 = prefix[i - 1].second;
		ans = max(ans, max(r2 - l2 + 1, 0ll) + max(r1 - l1 + 1, 0ll));
	}
	cout << ans;
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:32:44: error: 'inf' was not declared in this scope; did you mean 'int'?
   32 |         vector<pair<int, int>> suffix(n, { inf,-inf });
      |                                            ^~~
      |                                            int
a.cc:32:54: error: no matching function for call to 'std::vector<std::pair<long long int, long long int> >::vector(ll&, <brace-enclosed initializer list>)'
   32 |         vector<pair<int, int>> suffix(n, { inf,-inf });
      |                                                      ^
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:4:
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with <template-parameter-2-2> = _InputIterator; _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >]'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/stl_iterator_base_funcs.h:66,
                 from /usr/include/c++/14/string:47,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator_base_types.h: In substitution of 'template<class _InIter> using std::_RequireInputIter = std::__enable_if_t<((bool)std::is_convertible<typename std::iterator_traits< <template-parameter-1-1> >::iterator_category, std::input_iterator_tag>::value)> [with _InIter = long long int]':
/usr/include/c++/14/bits/stl_vector.h:705:9:   required from here
  705 |                typename = std::_RequireInputIter<_InputIterator>>
      |                ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator_base_types.h:252:57: error: no type named 'iterator_category' in 'struct std::iterator_traits<long long int>'
  252 |                                    input_iterator_tag>::value>;
      |                                                         ^~~~~
/usr/include/c++/14/bits/stl_vector.h:678:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; allocator_type = std::allocator<std::pair<long long int, long long int> >]'
  678 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:678:43: note:   no known conversion for argument 1 from 'll' {aka 'long long int'} to 'std::initializer_list<std::pair<long long int, long long int> >'
  678 |       vector(initializer_list<value_type> __l,
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:659:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, std::__type_identity_t<_Alloc>&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; std::__type_identity_t<_Alloc> = std::allocator<std::pair<long long int, long long int> >]'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:659:23: note:   no known conversion for argument 1 from 'll' {aka 'long long int'} to 'std::vector<std::pair<long long int, long long int> >&&'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |              ~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::false_type) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; allocator_type = std::allocator<std::pair<long long int, long long int> >; std::false_type = std::false_type]'
  640 |       vector(vector&& __rv, const allocator_type& __m, false_type)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:635:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::true_type) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; allocator_type = std::allocator<std::pair<long long int, long long int> >; std::true_type = std::true_type]'
  635 |       vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:635:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:624:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&, std::__type_identity_t<_Alloc>&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; std::__type_identity_t<_Alloc> = std::allocator<std::pair<long long int, long long int> >]'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:624:28: note:   no known conversion for argument 1 from 'll' {aka 'long long int'} to 'const std::vector<std::pair<long long int, long long int> >&'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |              ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >]'
  620 |       vector(vector&&) noexcept = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:601:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >]'
  601 |       vector(const vector& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:601:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:569:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const value_type&, const allocator_type&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; size_type = long unsigned int; value_type = std::pair<long long int, long long int>; allocator_type = std::allocator<std::pair<long long int, long long int> >]'
  569 |       vector(size_type __n, const value_type& __value,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:569:47: note:   no known conversion for argument 2 from '<brace-enclosed initializer list>' to 'const std::vector<std::pair<long long int, long long int> >::value_type&' {aka 'const std::pair<long long int, long long int>&'}
  569 |       vector(size_type __n, const value_type& __value,
      |                             ~~~~~~~~~~~~~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const allocator_type&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; size_type = long unsigned int; allocator_type = std::allocator<std::pair<long long int, long long int> >]'
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:51: note:   no known conversion for argument 2 from '<brace-enclosed initializer list>' to 'const std::vector<std::pair<long long int, long long int> >::allocator_type&' {aka 'const std::allocator<std::pair<long long int, long long int> >&'}
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |                             ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const allocator_type&) [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >; allocator_type = std::allocator<std::pair<long long int, long long int> >]'
  542 |       vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:531:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector() [with _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >]'
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:531:7: note:   candidate expects 0 arguments, 2 provided
a.cc:33:55: error: no matching function for call to 'std::vector<std::pair<long long int, long long int> >::vector(ll&, <brace-enclosed initializer list>)'
   33 |         vector<pair<int, int>> prefix(n, { inf, -inf });
      |                                                       ^
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with <template-parameter-2-2> = _InputIterator; _Tp = std::pair<long long int, long long int>; _Alloc = std::allocator<std::pair<long long int, long long int> >]'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_vect