foo.cc: In member function 'void node::build(int, int)':
foo.cc:39:55: warning: suggest parentheses around '+' inside '>>' [-Wparentheses]
   this -> left = new node; this -> left -> build(l, l + r >> 1);
                                                     ~~^~~
foo.cc:40:55: warning: suggest parentheses around '+' inside '>>' [-Wparentheses]
   this -> right = new node; this -> right -> build((l + r >> 1) + 1, r);
                                                     ~~^~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。