foo.cc: In function 'int main()':
foo.cc:10:14: warning: array subscript has type 'char' [-Wchar-subscripts]
    st[ ++ top] = ch[i];
              ^
foo.cc:13:21: warning: array subscript has type 'char' [-Wchar-subscripts]
    char tmp = st[top];
                     ^
foo.cc:6:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%s", ch);
  ~~~~~^~~~~~~~~~

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