Python实现二叉树的四种遍历

    对于一个没学过数据结构这门课程的编程菜鸟来说，自己能理解数据结构中的相关概念，但是自己动手通过Python，C++来实现它们却总感觉有些吃力。递归，指针，类这些知识点感觉自己应用的不够灵活，这是自己以后需要加强的地方。以下给出Python实现二叉树四种的遍历。

# -*- coding: utf-8 -*-

"""
Created on Mon Apr 03 19:58:58 2017

@author: Administrator
"""
class node(object):
    def __init__(self,data=None,left=None,right=None):
        self.data=data
        self.left=left
        self.right=right
       
#深度
def depth(tree):
    if tree==None:
        return 0
    left,right=depth(tree.left),depth(tree.right)
    return max(left,right)+1
#前序遍历   
def pre_order(tree):
    if tree==None:
        return
    print tree.data
    pre_order(tree.left)
    pre_order(tree.right)
#中序遍历   
def mid_order(tree):
    if tree==None:
        return
    mid_order(tree.left)
    print tree.data
    mid_order(tree.right)    
#后序遍历   
def post_order(tree):
    if tree==None:
        return
    post_order(tree.left)
    post_order(tree.right)   
    print tree.data
    
#层次遍历    
def level_order(tree):
     if tree==None:
        return 
     q=[]
     q.append(tree)
     while q:
         current=q.pop(0)
         print current.data
         if current.left!=None:
            q.append(current.left)
         if current.right!=None:
            q.append(current.right)
#按层次打印
def level2_order(tree):
     if tree==None:
        return 
     q=[]
     q.append(tree)
     results={}
     level=0
     current_level_num=1
     nextlevelnum=0
     d=[]
     while q: 
         current=q.pop(0)
         current_level_num-=1
         d.append(current.data)
         if current.left!=None:
            q.append(current.left) 
            nextlevelnum+=1
         if current.right!=None:
            q.append(current.right) 
            nextlevelnum+=1   
         if current_level_num==0:
            current_level_num=nextlevelnum
            nextlevelnum=0
            results[level]=d
            d=[]
            level+=1
     print results    
     
if __name__=='__main__':  
    tree=node('D',node('B',node('A'),node('C')),node('E',right=node('G',node('F'))))  
    print'前序遍历：'  
    pre_order(tree)  
    print('\n')  
    print('中序遍历：')  
    mid_order(tree)  
    print('\n')  
    print '后序遍历：'  
    post_order(tree)  
    print('\n')  
    print "层次遍历"
    level2_order(tree)