foo.c:4:6: error: variably modified 'ch' at file scope
 char ch[N], len, st[N], top = 0;
      ^~
foo.c:4:18: error: variably modified 'st' at file scope
 char ch[N], len, st[N], top = 0;
                  ^~
foo.c: In function 'main':
foo.c:10:6: warning: array subscript has type 'char' [-Wchar-subscripts]
    st[ ++ top] = ch[i];
      ^
foo.c:13:17: warning: array subscript has type 'char' [-Wchar-subscripts]
    char tmp = st[top];
                 ^
foo.c:6:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%s", ch);
  ^~~~~~~~~~~~~~~

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