Tree structure:
        7
    3
        6
            12
1
            11
        5
            10
    2
            9
        4
            8

preorder: 1 2 4 8 9 5 10 11 3 6 12 7
inorder: 8 4 9 2 10 5 11 1 12 6 3 7
postorder: 8 9 4 10 11 5 2 12 6 7 3 1