Language: C++

// #include <bits/stdc++.h>
#include <algorithm>
#include <cmath>
#include <deque>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
typedef long long ll;

#define all(x) begin(x), end(x)
#define rep(i,n) for(int i=0; i<n; i++)

struct atsu {int cnt; char c;};

int main() {
    string s;
    cin >> s;

    vector<atsu> v;
    int cnt = 0;
    char pre = ' ';
    rep(i, s.size()) {
        if (s[i]==pre) cnt++;
        else {
            v.push_back((atsu){cnt, pre});
            cnt = 1;
            pre = s[i];
        }
    }
    v.push_back((atsu){cnt, pre});

    int ans;
    if (v[0].c != '?') ans = v[0].cnt;
    for (int i=1; i<(int)v.size(); i++) {
        if (i+1==(int)v.size() && v[i].c=='?') break;
        if (v[i].c != '?') ans = max(ans, v[i].cnt);
        else if (v[i-1].c == v[i+1].c) {
            if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
            if (v[i].cnt %2 == 0) ans = max(ans, 2);
        }
    }

    cout << ans << endl;

    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:49:51: error: no match for 'operator<' (operand types are 'int' and '__gnu_cxx::__alloc_traits<std::allocator<atsu>, atsu>::value_type' {aka 'atsu'})
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
In file included from /usr/include/c++/14/deque:66,
                 from a.cc:4:
/usr/include/c++/14/bits/stl_deque.h:2334:5: note: candidate: 'template<class _Tp, class _Alloc> bool std::operator<(const deque<_Tp, _Alloc>&, const deque<_Tp, _Alloc>&)'
 2334 |     operator<(const deque<_Tp, _Alloc>& __x, const deque<_Tp, _Alloc>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_deque.h:2334:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::deque<_Tp, _Alloc>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/deque:81:
/usr/include/c++/14/tuple:2600:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator<(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2600 |     operator<(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2600:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::tuple<_UTypes ...>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 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:5:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/string_view:688:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  688 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:688:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   'atsu' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/basic_string.h:3874:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3874 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/basic_string.h:3888:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3888 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3888:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
/usr/include/c++/14/bits/basic_string.h:3901:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3901 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3901:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const _CharT*' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cnt < v[i+1]) v[i+1].cnt += v[i-1].cnt - v[i].cnt;
      |                                                          ^
In file included from /usr/include/c++/14/list:65,
                 from a.cc:7:
/usr/include/c++/14/bits/stl_list.h:2188:5: note: candidate: 'template<class _Tp, class _Alloc> bool std::operator<(const __cxx11::list<_Tp, _Alloc>&, const __cxx11::list<_Tp, _Alloc>&)'
 2188 |     operator<(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_list.h:2188:5: note:   template argument deduction/substitution failed:
a.cc:49:58: note:   mismatched types 'const std::__cxx11::list<_Tp, _Alloc>' and 'int'
   49 |             if (v[i].cnt < v[i-1].cnt && v[i].cn