foo.cc: In function 'int main()':
foo.cc:14:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::stack<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   14 |  for(int i=0;i<st.size();i++){
      |              ~^~~~~~~~~~
foo.cc:15:13: error: no match for 'operator[]' (operand types are 'std::stack<int>' and 'int')
   15 |   char c =st[i];
      |             ^

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