foo.cc: In function 'int main()':
foo.cc:38:38: warning: array subscript has type 'char' [-Wchar-subscripts]
         in_nodes[len++] = tmp_nodes[c] = tmp;
                                      ^
foo.cc:42:39: warning: array subscript has type 'char' [-Wchar-subscripts]
         pos_nodes[len++] = tmp_nodes[c];
                                       ^