/in/foo.cc: In function 'int main()':
/in/foo.cc:13:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   13 |    num[a[j]]++;
      |            ^
/in/foo.cc:52:35: warning: 'rec' may be used uninitialized in this function [-Wmaybe-uninitialized]
   52 |    if (lena % 2 != 0) sub2 = sub2 + rec + sub1;
      |                              ~~~~~^~~~~