Language: C++

#include <unordered_set>
#include <iostream>
#include <algorithm>

bool is_in_indices(int a, int b, std::unordered_set<int>* indices) {
  char key[6];
  sprintf(key, "%d_%d", a, b);
  return indices->count(std::string(key) > 0;
}

void walk(int cx, int tx, int cy, int ty, std::unordered_set<std::string>* indices, std::vector<std::string>* result) {
  if (cx == tx && cy == ty) {
    return;
  }
  if (cx < tx) {
    if (is_in_indices(cx + 1, cy, indices)) {
      result->push_back("R");
      return walk(cx + 1, tx, cy, ty, indices, result);
    }
  }
  if (cy < ty) {
    if (is_in_indices(cx, cy + 1, indices)) {
      result->push_back("D");
      return walk(cx, tx, cy + 1, ty, indices, result);
    }
  }
  if (cx > tx) {
    if (is_in_indices(cx - 1, cy, indices)) {
      result->push_back("L");
      return walk(cx - 1, tx, cy, ty, indices, result);
    }
  }
  if (cy > ty) {
    if (is_in_indices(cx cy - 1, indices)) {
      result->push_back("U");
      return walk(cx tx, cy - 1, ty, indices, result);
    }
  }

  if (is_in_indices(cx + 1, cy, indices)) {
    result->push_back("R");
    return walk(cx tx, cy - 1, ty, indices, result);
  }
  if (is_in_indices(cx - 1, cy, indices)) {
    result->push_back("L");
    return walk(cx tx, cy - 1, ty, indices, result);
  }
  if (is_in_indices(cx, cy + 1, indices)) {
    result->push_back("D");
    return walk(cx tx, cy - 1, ty, indices, result);
  }
  if (is_in_indices(cx, cy - 1, indices)) {
    result->push_back("U");
    return walk(cx tx, cy - 1, ty, indices, result);
  }
}

int main() {
  int sx = 0;
  int sy = 0;
  int tx = 0;
  int ty = 0;
  std::cin >> sx;
  std::cin >> sy;
  std::cin >> tx;
  std::cin >> ty;
  
  std::unordered_set<int> indices;
  std::vector<std::string> result;
  
  walk(sx, sy, tx, ty, &indices, &result);
  walk(tx, ty, sx, sy, &indices, &result);
  walk(sx, sy, tx, ty, &indices, &result);
  walk(tx, ty, sx, sy, &indices, &result);
  
  for (auto p&: result) {
    printf("%s", p.c_str());
  }
  printf("\n");
}

Compiler output:
a.cc: In function 'bool is_in_indices(int, int, std::unordered_set<int>*)':
a.cc:8:42: error: no match for 'operator>' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'int')
    8 |   return indices->count(std::string(key) > 0;
      |                              ~~~~~~~~~~~ ^ ~
      |                              |             |
      |                              |             int
      |                              std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/tuple:38,
                 from /usr/include/c++/14/bits/hashtable_policy.h:34,
                 from /usr/include/c++/14/bits/hashtable.h:35,
                 from /usr/include/c++/14/bits/unordered_set.h:33,
                 from /usr/include/c++/14/unordered_set:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_pair.h:1058:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator>(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1058 |     operator>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1058:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/tuple:2619:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator>(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2619 |     operator>(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2619:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::tuple<_UTypes ...>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/bits/hashtable_policy.h:36:
/usr/include/c++/14/bits/stl_iterator.h:462:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator>(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  462 |     operator>(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:462:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/stl_iterator.h:507:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator>(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  507 |     operator>(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:507:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/stl_iterator.h:1714:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator>(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1714 |     operator>(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1714:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/stl_iterator.h:1774:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator>(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1774 |     operator>(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1774:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
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:2:
/usr/include/c++/14/string_view:695:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  695 |     operator> (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:695:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/string_view:702:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  702 |     operator> (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:702:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/string_view:710:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator>(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  710 |     operator> (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:710:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/basic_string.h:3915:5: 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>&)'
 3915 |     operator>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3915:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/basic_string.h:3929:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator>(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3929 |     operator>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3929:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   mismatched types 'const _CharT*' and 'int'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
/usr/include/c++/14/bits/basic_string.h:3942:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator>(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3942 |     operator>(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3942:5: note:   template argument deduction/substitution failed:
a.cc:8:44: note:   mismatched types 'const _CharT*' and 'std::__cxx11::basic_string<char>'
    8 |   return indices->count(std::string(key) > 0;
      |                                            ^
a.cc: At global scope:
a.cc:11:85: error: 'std::vector' has not been declared
   11 | void walk(int cx, int tx, int cy, int ty, std::unordered_set<std::string>* indices, std::vector<std::string>* result) {
      |                                                                                     ^~~
a.cc:11:96: error: expected ',' or '...' before '<' token
   11 | void walk(int cx, int tx, int cy, int ty, std::unordered_set<std::string>* indices, std::vector<std::string>* result) {
      |                                                                                                ^
a.cc: In function 'void walk(int, int, int, int, std::unordered_set<std::__cxx11::basic_string<char> >*, int)':
a.cc:16:35: error: cannot convert 'std::unordered_set<std::__cxx11::basic_string<char> >*' to 'std::unordered_set<int>*'
   16 |     if (is_in_indices(cx + 1, cy, indices)) {
      |                                   ^~~~~~~
      |                                   |
      |                                   std::unordered_set<std::__cxx11::basic_string<char> >*
a.cc:5:59: note:   initializing argument 3 of 'bool is_in_indices(int, int, std::unordered_set<int>*)'
    5 | bool is_in_indices(int a, int b, std::unordered_set<int>* indices) {
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
a.cc:17:7: error: 'result' was not declared in this scope
   17 |       result->push_back("R");
      |       ^~~~~