Language: C++

#include<bits/stdc++.h>
using namespace std;
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define ll long long
#define mem(x,y) memset(x,y,sizeof(x)) //memset array
#define fo(i,r,k)  for((i) = r; (i) < (k); i=i+1)
#define fok(i,k,n) for((i) = k; (i) > n; i=i-1)
#define deb(x) cout << #x << " = " << x << endl
#define deb2(x, y) cout << #x << " = " << x << ", " << #y << " = " << y << endl
#define deb3(x, y, z) cout << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << endl
#define bug puts("Hello");
#define mp make_pair
#define pb push_back
#define pi pair<int,int>
#define pl pair<long long,long long>
#define vi vector<int>
#define vl vector<long long>
#define vpl vector<pair<long long,long long>>
#define vpi vector<pair<int,int>>
#define all(x) x.begin(),x.end()
#define f first
#define s second
#define br cout<<"\n";
#define sortall(x) sort(x.begin(),x.end())
#define modexpo poww
ll modexpo(ll x,ll y,ll m){x=x%m;if(y==0){return 1;}if(y%2==0){return modexpo(x*x,y/2,m); }else{return (x*modexpo(x*x,y/2,m))%m; }}
void io();
const int mod  = 1e9+7;
const int maxn = 2e5 + 10;
string s1,s2;
int n1,n2;
string dp[3001][3001];
int rec(int n1,int n2)
{
    if(n1<0 || n2<0) return 0;
    if(dp[n1][n2] != -1) return dp[n1][n2];
    if(s1[n1] == s2[n2])
    {
        int ans = rec(n1-1,n2-1) + 1;
        dp[n1][n2] = ans;
        return ans;
    }
    else {
        int ans1 = rec(n1-1,n2) ;
        int ans2 = rec(n1,n2-1) ;
        if(ans1 > ans2) {
            dp[n1][n2] = ans1;
        }
        else {
            dp[n1][n2] = ans2;
        }
        return dp[n1][n2];
    }
}
string solve(int n1,int n2)
{
    string s = ""; 
  int i = n1-1 , j = n2-1; 
  while(i>-1 && j>-1)
  {
    if(s1[i-1] == s2[j-1])
    {
      s.push_back(s1[i-1]); 
      i--; j--; 
    }
    else if(dp[i-1][j] > dp[i][j-1])
    {
      i--; 
    }
    else
    {
      j--; 
    }
  }
  reverse(s.begin(),s.end());
  return s; 
}
int main()
{
int i,j;
cin>>s1>>s2;
n1 = s1.size();
n2 = s2.size();
mem(dp,-1); 
rec(n1,n2); 
cout<<solve(n1,n2); 
return 0;
}

void io()
{
#ifndef ONLINE_JUDGE
freopen("input.txt","r",stdin);
freopen("output.txt","w",stdout);
#endif
}


Compiler output:
a.cc: In function 'int rec(int, int)':
a.cc:36:19: error: no match for 'operator!=' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'int')
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |        ~~~~~~~~~~ ^~ ~~
      |                 |    |
      |                 |    int
      |                 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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::match_results<_BiIter, _Alloc>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/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:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1753 |     operator!=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note:   template argument deduction/substitution failed:
a.cc:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
In file included from /usr/include/c++/14/bits/char_traits.h:42,
                 from /usr/include/c++/14/string:42,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/bits/postypes.h:197:5: note: candidate: 'template<class _StateT> bool std::operator!=(const fpos<_StateT>&, const fpos<_StateT>&)'
  197 |     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:197:5: note:   template argument deduction/substitution failed:
a.cc:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::fpos<_StateT>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
In file included from /usr/include/c++/14/string:43:
/usr/include/c++/14/bits/allocator.h:243:5: note: candidate: 'template<class _T1, class _T2> bool std::operator!=(const allocator<_CharT>&, const allocator<_T2>&)'
  243 |     operator!=(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:243:5: note:   template argument deduction/substitution failed:
a.cc:36:23: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::allocator<_CharT>'
   36 |     if(dp[n1][n2] != -1) return dp[n1][n2];
      |                       ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:651:5: note: candidate: 'template<cl