def breadth_travel(self, root):
    """利用队列实现树的层次遍历"""
    if root == None:
      return
    queue = []
    queue.append(root)
    while queue:
      node = queue.pop(0)
      print node.elem,
      if node.lchild != None:
        queue.append(node.lchild)
      if node.rchild != None:
        queue.append(node.rchild)

 #实现 1
 def preorder(self, root):
    """递归实现先序遍历"""
    if root == None:
      return
    print root.elem
    self.preorder(root.lchild)
    self.preorder(root.rchild)

 #实现 2
 def depth_tree(tree_node):
   if tree_node is not None:
     print (tree_node._data)
     if tree_node._left is noe None:
       return depth_tree(tree_node._left)
     if tree_node._right is not None:
       return depth_tree(tree_node._right)

def inorder(self, root):
   """递归实现中序遍历"""
   if root == None:
     return
   self.inorder(root.lchild)
   print root.elem
   self.inorder(root.rchild)

def postorder(self, root):
   """递归实现后续遍历"""
   if root == None:
     return
   self.postorder(root.lchild)
   self.postorder(root.rchild)
   print root.elem