foo.cc: In function 'int main()':
foo.cc:18:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   18 |  for (i = 0; i < s1.size(); i++)
      |              ~~^~~~~~~~~~~
foo.cc:24:11: warning: suggest explicit braces to avoid ambiguous 'else' [-Wdangling-else]
   24 |   else if (s1[i] == ')')
      |           ^
foo.cc:36:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::stack<Do>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   36 |  for (i = 0; i <= k.size(); i++)
      |              ~~^~~~~~~~~~~
foo.cc:39:10: warning: comparison of integer expressions of different signedness: 'int' and 'std::stack<Do>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   39 |    if (i != k.size())
      |        ~~^~~~~~~~~~~
foo.cc:16:7: warning: unused variable 'm' [-Wunused-variable]
   16 |  char m;
      |       ^

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