前序遍歷：根、左、右
中序遍歷：左、根、右
后序遍歷：左、右、根
代碼：

# 前序遍歷
def preorder(tree):
    if tree:
        print(tree.getRootVal())
    preorder(tree.getLeftChildren())
    preorder(tree.getRightChildren())

# 中序遍歷
def inorder(tree):
    if tree:
        inorder(tree.getLeftChildren())
        print(tree.getRootVal())
        preorder(tree.getRightChildren())

# 后序遍歷
def postorder(tree):
    if tree:
        postorder(tree.getLeftChildren())
        postorder(tree.getRightChildren())
        print(tree.getRootVal())