foo.cc:1:9: warning: #pragma once in main file
    1 | #pragma once
      |         ^~~~
foo.cc: In function 'int parentheses()':
foo.cc:88:11: warning: suggest explicit braces to avoid ambiguous 'else' [-Wdangling-else]
   88 |   else if (ch == ')')
      |           ^
foo.cc: In function 'int main()':
foo.cc:125:6: warning: unused variable 'n' [-Wunused-variable]
  125 |  int n;
      |      ^
foo.cc: In function 'int parentheses()':
foo.cc:77:9: warning: control reaches end of non-void function [-Wreturn-type]
   77 |  string s1;
      |         ^~

[Hydro](https://hydro.ac)提供评测服务