深度优先过程
def depth_tree(tree_node):
if tree_node is not None:
print（tree_node._data）
if tree_node._left is not None:
return depth_tree(tree_node._left)
if tree_node._right is not None:
return depth_tree(tree_node._right)
广度优先过程
def level_queue(root):
"""
利用队列实现树的广度优先遍历
"""
if root is None：
return
my_queue = []
node = root
my_queue.append(node)
while my_queue:
node = my_queue.pop(0)
print(node.element)
if node.lchild is not None:
my_queue.append(node.lchild)
if node.rchild is not None:
my_queue.append(node.rchild)