Heap sort unstable example
Consider array 21 20a 20b 12 11 8 7 (already in max-heap format)
here 20a = 20b just to differentiate the order we represent them as 20a and 20b
While heapsort first 21 is removed and placed in the last index then 20a is removed and placed in last but one index and 20b in the last but two index so after heap sort the array looks like
7 8 11 12 20b 20a 21.
It does not preserve the order of elements and hence can’t be stable