Language: C++

#include <bits/stdc++.h>
using namespace std;

#define endl "\n"

typedef long long ll;
typedef pair <ll, ll> pii;
typedef long double ld;

const ll DIM = 100007;
const ll INF = 1000000007;
ll n, m, k;
ll a, b, c;
ll v[DIM+7], w[DIM+7];
string used[3007][3007];
string s, s1;

string f(ll a,ll b){
    if(a==-1||b==-1)return "";
    if(used[a][b]!="-")return used[a][b];
    string ans="",ans1="",ans2="";
if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
if(ans1.size()<ans2.size())swap(ans1,ans2);
if(ans.size()<ans1.size())swap(ans1,ans);
used[a][b]=ans;
k++;
return ans;
}

int main()
{
    for(ll i=0;i<3007;i++)
    for(ll j=0;j<3007;j++)
        used[i][j]="-";
	ios_base::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	cin >> s >> s1;
	cout<<f(s.size()-1,s1.size()-1);
    //cout<<endl<<k;
}


Compiler output:
a.cc: In function 'std::string f(ll, ll)':
a.cc:22:57: error: no match for 'operator!=' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'char')
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                             ~~~~~~~~~~~~^~~~~
      |                                                        |  |
      |                                                        |  char
      |                                                        std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1132:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator!=(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1132 |     operator!=(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1132:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1212:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator!=(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1212 |     operator!=(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1212:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'char'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1305:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator!=(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1305 |     operator!=(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1305:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1379:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator!=(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1379 |     operator!=(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1379:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'char'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1473:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator!=(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1473 |     operator!=(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1473:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1547:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator!=(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1547 |     operator!=(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1547:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'char'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:1647:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator!=(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1647 |     operator!=(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1647:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/regex.h:2213:5: note: candidate: 'template<class _Bi_iter, class _Alloc> bool std::__cxx11::operator!=(const match_results<_BiIter, _Alloc>&, const match_results<_BiIter, _Alloc>&)'
 2213 |     operator!=(const match_results<_Bi_iter, _Alloc>& __m1,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:2213:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::match_results<_BiIter, _Alloc>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1052:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator!=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1052 |     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1052:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:455:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  455 |     operator!=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:455:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  500 |     operator!=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1686 |     operator!=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note:   template argument deduction/substitution failed:
a.cc:22:59: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   22 | if(s[a]==s1[b])ans=f(a-1,b-1)+s[a];else {if(used[a-1][b]!='-')ans1=used[a-1][b];else ans1=f(a-1,b);if(used[a][b-1]!='-')ans2=used[a][b-1];else ans2=f(a,b-1);}
      |                                                           ^~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note: candidate: 'template<class _Iterator> con