Page 355, Exercise 2 - Heap Sort

Assume that our initial array contains the values: 90 80 90 70 60. The initial heap would be as follows:

                                90a 
                              /     \ 
                             80     90b 
                            /  \ 
                           70  60 
 

The first pass of the sort places the first 90 in the last array position.  The second 90 will appear earlier in the sorted array.  Hence, the sort is unstable.