A particularly simple and beautiful implementation of the heap
structure is the implicit heap. Data is simply put into an array 
and the childrens of element x[i] are defined to be the
elements x[2i] and x[2i+1]. Thus the parent of the element c can
be found at c/2 (integer division).
With this structure the heap uses absolutely no space beyond what is needed to store the elements, and the shape property is guaranteed to hold.