foo.cc: In function ‘int main()’:
foo.cc:8:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |         for(int i=1;i<strlen(s);i++)
      |                     ~^~~~~~~~~~
foo.cc:9:22: warning: array subscript has type ‘char’ [-Wchar-subscripts]
    9 |                 a[s[i]]++;
      |                   ~~~^
foo.cc:12:22: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   12 |                 if(a[i]!=0)
      |                      ^