Given an array 1 to N , how many permutations of it will be Heap of N! possible permutations.

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Approach:

Recurrence equation:
T(n) = T(k) * T(n-k-1) * (n-1)Ck where k = number of nodes on left

We always have to take care that the Max-Height(left-subtree) - Max-Height(right-subtree) <= 1.

number of leafs in a heap of size n = Math.floor((n+1)/2).

T(1) = 1
T(2) = 1
T(3) = 2

T(4) = 3C2 * T(2) * T(1) = 3
T(5) = 4C3 * T(3) * T(1) = 8
T(6) = 5C3 * T(3) * T(2) = 20
T(7) = 6C3 * T(3) * T(3) = 80

T(8) = 7C4 * T(4) * T(3) = 210
T(9) = 8C5 * T(5) * T(3) = 896
T(10) = 9C6 * T(6) * T(3) = 3360
T(11) = 10C7 * T(7) * T(3) = 19200
T(12) = 11C7 * T(7) * T(4) = 79200
T(13) = 12C7 * T(7) * T(5) = 506880
T(14) = 13C7 * T(7) * T(6) = 2745600
T(15) = 14C7 * T(7) * T(7) = 21964800

[참고] 

http://karmaandcoding.blogspot.kr/2012/02/number-of-min-heaps-from-array-of-size.html