create a binary tree
node * root = getnode ( 20 ) ; root -> left = getnode ( 10 ) ; root -> right = getnode ( 30 ) ; root -> left -> left = getnode ( 5 ) ; root -> left -> right = getnode ( 15 ) ; root -> left -> right -> left = getnode ( 12 ) ; root -> right -> left = getnode ( 25 ) ; root -> right -> right = getnode ( 40 ) ;