Language: C++

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
int main()
{
	string s,t;
	cin>>s>>t;
	int m=s.size();
	int n=t.size();
	pair <int,int> dpmat[m+1][n+1];
	for(i=0;i<m;i++)
	{
		dpmat[i][0].first=0;
		dpmat[i][0].second=0;
	}
	for(i=0;i<n;i++)
	{
		dpmat[0][i].first=0;
		dpmat[0][i].second=0;
	}	
	for(i=1;i<=m;i++)
	{
		for(j=1;j<=n;j++)
		{
			dpmat[i][j].first=max(dpmat[i][j-1].first,dpmat[i-1][j].first);
			dpmat[i][j].first=max(dpmat[i][j].first,dpmat[i-1][j-1].first+(s[i-1]==t[j-1]));
			if(dpmat[i][j].first==dpmat[i][j-1].first)
				dpmat[i][j].second=3;
			else if(dpmat.first[i][j]==dpmat.first[i-1][j])
				dpmat[i][j].second=1;
			else// if(dpmat.first[i][j]==dpmat.first[i-1][j])
				dpmat[i][j].second=2;
		}
	}
	if(dpmat[m][n]==0)
	{
		cout<<"";
		return;
	}
	i=m;j=n;
	vector<char> ans;
	while(1)
	{
		if(dpmat[i][j].second==1)
		{
			i--;
		}
		else if(dpmat[i][j].second==3)
		{
			j--;
		}
		else if(dpmat[i][j].second==2)
		{
			if(s[i-1]==t[j-1])
				ans.pb(s[i-1]);
			i--;j--;
		}
		else
			break;
	}
	for (i = 0; i < v.size(); i++)
		cout << v.pop_back();

}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:13: error: 'i' was not declared in this scope
   11 |         for(i=0;i<m;i++)
      |             ^
a.cc:16:13: error: 'i' was not declared in this scope
   16 |         for(i=0;i<n;i++)
      |             ^
a.cc:21:13: error: 'i' was not declared in this scope
   21 |         for(i=1;i<=m;i++)
      |             ^
a.cc:23:21: error: 'j' was not declared in this scope
   23 |                 for(j=1;j<=n;j++)
      |                     ^
a.cc:29:39: error: request for member 'first' in 'dpmat', which is of non-class type 'std::pair<int, int> [(m + 1)][(n + 1)]'
   29 |                         else if(dpmat.first[i][j]==dpmat.first[i-1][j])
      |                                       ^~~~~
a.cc:29:58: error: request for member 'first' in 'dpmat', which is of non-class type 'std::pair<int, int> [(m + 1)][(n + 1)]'
   29 |                         else if(dpmat.first[i][j]==dpmat.first[i-1][j])
      |                                                          ^~~~~
a.cc:35:23: error: no match for 'operator==' (operand types are 'std::pair<int, int>' and 'int')
   35 |         if(dpmat[m][n]==0)
      |            ~~~~~~~~~~~^~~
      |                      |  |
      |                      |  int
      |                      std::pair<int, int>
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:1103:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1103 |     operator==(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1103:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1199:5: 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>&)'
 1199 |     operator==(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1199:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1274:5: 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>&)'
 1274 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1274:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1366:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1366 |     operator==(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1366:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1441:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1441 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1441:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1534:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1534 |     operator==(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1534:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:1613:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1613 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1613:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/regex.h:2186:5: note: candidate: 'template<class _Bi_iter, class _Alloc> bool std::__cxx11::operator==(const match_results<_BiIter, _Alloc>&, const match_results<_BiIter, _Alloc>&)'
 2186 |     operator==(const match_results<_Bi_iter, _Alloc>& __m1,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:2186:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::__cxx11::match_results<_BiIter, _Alloc>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
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:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
   35 |         if(dpmat[m][n]==0)
      |                         ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::reverse_iterator<_Iterator>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::reverse_iterator<_Iterator>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::move_iterator<_IteratorL>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::move_iterator<_IteratorL>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
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:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pair<int, int>' is not derived from 'const std::fpos<_StateT>'
   35 |         if(dpmat[m][n]==0)
      |                         ^
In file included from /usr/include/c++/14/string:43:
/usr/include/c++/14/bits/allocator.h:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:35:25: note:   'std::pai