/in/foo.cc: In function 'int main()':
/in/foo.cc:22:56: error: 'lower_bound' was not declared in this scope
                   int y=lower_bound(a+1,a+top+1,len-x+1)-a;
                                                        ^
/in/foo.cc:22:56: note: suggested alternative:
In file included from /usr/include/c++/6/algorithm:62:0,
                 from /in/foo.cc:3:
/usr/include/c++/6/bits/stl_algo.h:2020:5: note:   'std::lower_bound'
     lower_bound(_ForwardIterator __first, _ForwardIterator __last,
     ^~~~~~~~~~~