Language: C++

#include <bits/stdc++.h>
#define PREP(i, s, x) for(ll i = (s); i < (x); i++)
#define MREP(i, s, x) for(ll i = (s); i >= (x); i--)
#define REP(i, x) PREP(i, 0, x)
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
template<typename T> inline bool chmax(T &a, T b) { return ((a < b) ? (a = b, true) : (false)); }
template<typename T> inline bool chmin(T &a, T b) { return ((a > b) ? (a = b, true) : (false)); }

// variable -------------------------------------------

constexpr ll INF = (1ll << 59);
constexpr ll MOD = 1000000007ll;

// structure ------------------------------------------

// function -------------------------------------------

// main -----------------------------------------------

int main() {
	ll n, q;
	cin >> n >> q;
	vector<ll> c(n);
	REP(i, n){
		cin>> c[i];
		c[i] --;
	}
	vector<pair<P, ll>> lr(q);
	REP(i, q){
		ll l, r;
		cin >> l >> r;
		lr[i] = make_pair(make_pair(l, r), i);
	}

	sort(lr.begin(), lr.end());

	vector<priority_queue<ll>> pq(n);
	MREP(i, n-1, 0){
		pq[c[i]].push(i);
	}

	set<ll> nextball;

	REP(i, n){
		if(pq[i].empty() == true){
			continue;
		}
		nextball.insert(pq[i].top());
		pq[i].pop();
	}

	vector<ll> ans(q);
	ll now_index = 0;
	REP(i, q){
		for(ll j = now_index + 1; j <= lr[i].first.first; j ++){
			nextball.erase(j-1);
			if(pq[c[j-1]].empty() == true){
				continue;
			}
			nextball.insert(pq[c[j-1]].top());
			pq[c[j-1]].pop();
		}
		auto ite = nextball.upper_bound(lr[i].first.second);
		ll(ite - nextball.begin());
	}

	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:66:24: error: no match for 'operator-' (operand types are 'std::_Rb_tree_const_iterator<long long int>' and 'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'})
   66 |                 ll(ite - nextball.begin());
      |                    ~~~ ^ ~~~~~~~~~~~~~~~~
      |                    |                   |
      |                    |                   _Rb_tree_const_iterator<[...]>
      |                    _Rb_tree_const_iterator<[...]>
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:618:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__y.base() - __x.base())) std::operator-(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  618 |     operator-(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:618:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::reverse_iterator<_Iterator>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__x.base() - __y.base())) std::operator-(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1790 |     operator-(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::move_iterator<_IteratorL>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/complex:370:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const complex<_Tp>&)'
  370 |     operator-(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:370:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::complex<_Tp>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/complex:379:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const _Tp&)'
  379 |     operator-(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:379:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::complex<_Tp>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/complex:388:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const _Tp&, const complex<_Tp>&)'
  388 |     operator-(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:388:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'} is not derived from 'const std::complex<_Tp>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/complex:465:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&)'
  465 |     operator-(const complex<_Tp>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:465:5: note:   candidate expects 1 argument, 2 provided
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:406:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::valarray<_Tp>'
   66 |                 ll(ite - nextball.begin());
      |                                         ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
a.cc:66:41: note:   'std::_Rb_tree_const_iterator<long long int>' is not derived from 'const std::valarray<_Tp>'
   66 |                 ll(ite - nextball.begin());