Language: C++

#include <iostream>
#include <cmath>
using namespace std;
 
#define MAXN 3010
 
string s,t;
string ans;
 
int dp[MAXN][MAXN];
 
void dfs(int i, int j) {
  int val = dp[i][j];
  
  if (val == 0) return;
  
  if (i == 0 && j == 0) {
    ans += (string)s[0];
    return;
  }
  
  if (i == 0) {
    ans += (string)s[0];
    return;
  }
  
  if (j == 0) {
    ans += (string)t[0];
  }
  if (val == dp[i-1][j]) {
    dfs(i-1,j);
  } else if (val == dp[i][j-1]) {
    dfs(i,j-1);
  } else {
	ans += (string)s[i];
    dfs(i-1,j-1);
  }
}
 
int main() {
	cin >> s >> t;
  	int n = (int)s.length(), m = (int)t.length();
  	for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        dp[i][j] = -1;
      }
    }
  	if (s[0] == t[0]) {
      dp[0][0] = 1;
    } else {
      dp[0][0] = 0;
    }
  	for (int i = 1; i < n; i++) {
      dp[i][0] = max(dp[i-1][0],(s[i]==t[0]?1:0));
    } 
  	for (int i = 1; i < m; i++) {
      dp[0][i] = max(dp[0][i-1],(s[0]==t[i]?1:0));
    }
  	for (int i = 1; i < n; i++) {
      for (int j = 1; j < m; j++) {
        dp[i][j] = max(dp[i-1][j],dp[i][j-1]);
        
        if (s[i] == t[j]) dp[i][j] = max(dp[i][j],1+dp[i-1][j-1]);
      }
    }
  	dfs(n-1,m-1);
  	for (int i = ans.length()-1; i >= 0; i--) {
      cout << ans[i];
    }
  cout << endl;
}

Compiler output:
a.cc: In function 'void dfs(int, int)':
a.cc:18:23: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)'
   18 |     ans += (string)s[0];
      |                       ^
In file included 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:1:
/usr/include/c++/14/bits/basic_string.h:800:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  800 |         basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:800:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/move.h:37,
                 from /usr/include/c++/14/bits/exception_ptr.h:41,
                 from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/ios:41:
/usr/include/c++/14/type_traits: In substitution of 'template<bool _Cond, class _Tp> using std::enable_if_t = typename std::enable_if::type [with bool _Cond = false; _Tp = void]':
/usr/include/c++/14/bits/basic_string.h:149:8:   required by substitution of 'template<class _CharT, class _Traits, class _Alloc> template<class _Tp, class _Res> using std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv = std::enable_if_t<((bool)std::__and_<std::is_convertible<const _Tp&, std::basic_string_view<_CharT, _Traits> >, std::__not_<std::is_convertible<const _Tp*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>*> >, std::__not_<std::is_convertible<const _Tp&, const _CharT*> > >::value), _Res> [with _Tp = char; _Res = void; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  149 |         using _If_sv = enable_if_t<
      |               ^~~~~~
/usr/include/c++/14/bits/basic_string.h:797:30:   required from here
  797 |       template<typename _Tp, typename = _If_sv<_Tp, void>>
      |                              ^~~~~~~~
/usr/include/c++/14/type_traits:2711:11: error: no type named 'type' in 'struct std::enable_if<false, void>'
 2711 |     using enable_if_t = typename enable_if<_Cond, _Tp>::type;
      |           ^~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:788:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, size_type, size_type, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  788 |         basic_string(const _Tp& __t, size_type __pos, size_type __n,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:788:9: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:765:9: note: candidate: 'template<class _InputIterator, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&) [with <template-parameter-2-2> = _InputIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  765 |         basic_string(_InputIterator __beg, _InputIterator __end,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:765:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:669:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(size_type, _CharT, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  669 |       basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:669:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:646:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  646 |       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:646:7: note:   template argument deduction/substitution failed:
a.cc:18:23: note:   cannot convert 's.std::__cxx11::basic_string<char>::operator[](0)' (type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}) to type 'const char*'
   18 |     ans += (string)s[0];
      |                       ^
/usr/include/c++/14/bits/basic_string.h:721:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  721 |       basic_string(basic_string&& __str, const _Alloc& __a)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:721:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:716:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  716 |       basic_string(const basic_string& __str, const _Alloc& __a)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:716:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:711:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::initializer_list<_Tp>, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  711 |       basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:711:45: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::initializer_list<char>'
  711 |       basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:682:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  682 |       basic_string(basic_string&& __str) noexcept
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:682:35: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::__cxx11::basic_string<char>&&'
  682 |       basic_string(basic_string&& __str) noexcept
      |                    ~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:624:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  624 |       basic_string(const _CharT* __s, size_type __n,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:624:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:604:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  604 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:604:7: note:   candidate expects 4 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:586:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  586 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:586:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:569:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  569 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:569:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:552:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  552 |       basic_string(const basic_string& __str)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:552:40: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::__cxx11::basic_string<char>&'
  552 |       basic_string(const basic_string& __str)
      |                    ~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:540:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Alloc&) [