Input:
Inorder traversal in[] = {4, 2, 5, 1, 3, 6}
Preorder traversal pre[] = {1, 2, 4, 5, 3, 6}

Output:
Postorder traversal is {4, 5, 2, 6, 3, 1}
Trversals in the above example represents following tree

         1
      /    \    
     2       3
   /   \      \
  4     5      6