Spiral Order Traversal of a Tree [Binary Tree Zigzag Level Order Traversal]

Problem statement:
Write a function to print spiral order traversal of a tree. 
For example.

Input - 
10
J
H
I
A
C
D
F
E
B
G
where 10 is the number of elements in the input array.

Output -
A
BC
FED
GHIJ