mirrored (t1,t2) = t1.data=t2.data && mirrored(t1.left, t2.right) && mirrored(t1.right, t2.left) inspired from : http://stackoverflow.com/questions/1482822/how-can-we-find-given-two-binary-trees-that-they-are-equal and http://www.glassdoor.com/Interview/Design-and-implement-an-algorithm-to-determine-if-a-binary-tree-is-symmetric-QTN_33466.htm