To find maximum possible height with 20 nodes, we need to find minimum no. of nodes of a given height
H(x) -- for height x
H(0) = nodes of nodes 1
H(1) = 2
H(2) = 1 +H(0) + H(1) = 4
H(3) = 1 + 4+ 2= 7
H(4) = 1 + 7 + 4 = 12
H(5) = 1+12+7 =20
So, maximum possible height for 20 nodes --> 5