#include <algorithm>
  bool is_heap( iterator start, iterator end );
  bool is_heap( iterator start, iterator end, StrictWeakOrdering cmp );

The is_heap() function returns true if the given range [start,end) is a heap.

If the strict weak ordering comparison function object cmp is given, then it is used instead of the < operator to compare elements.

is_heap() runs in linear time.