Suppose consider 15 elements 1,2,3,4,....15.
It is min heap ,level by level elements are stored.Root is at 1st level.
1st level; 1
2nd level: 2,3
3rd level: 4,5,6,7
4th level: 8,9,10,11,12,13,14,15.
In the second level elements are nodes 2,3 occupies 2 ways.no matter because it satisfies the heap property.
In the 3rd level also same nodes are 4,5,6,7 can be arranged in 4! ways.
In the 4th level 8!
SO TOTAL NO OF TREES ARE 1!*2!*4!*8*=1935360