python实现二叉树的前、中、后序遍历及按层遍历

需求：

python代码实现二叉树的：

1. 前序遍历，打印出遍历结果

2. 中序遍历，打印出遍历结果

3. 后序遍历，打印出遍历结果

4. 按树的level遍历，打印出遍历结果

5. 结点的下一层如果没有子节点，以‘N’代替

方法：

使用defaultdict或者namedtuple表示二叉树

使用StringIO方法，遍历时写入结果，最后打印出结果

打印结点值时，如果为空，StringIO（）写入‘N ’

采用递归访问子节点

代码

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

# test tree as below:
'''
1
/   \
/     \
/       \
/         \
2           3
/ \         / \
/   \       /   \
4    5     6     N
/ \  / \   / \
7  N N   N 8   9
/ \        / \  / \
N  N      N   N N N
'''

from collections import namedtuple
from io import StringIO

#define the node structure
Node = namedtuple('Node', ['data', 'left', 'right'])
#initialize the tree
tree = Node(1,
Node(2,
Node(4,
Node(7, None, None),
None),
Node(5, None, None)),
Node(3,
Node(6,
Node(8, None, None),
Node(9, None, None)),
None))
#read and write str in memory
output = StringIO()

#read the node and write the node's value
#if node is None, substitute with 'N '
def visitor(node):
if node is not None:
output.write('%i ' % node.data)
else:
output.write('N ')

#traversal the tree with different order
def traversal(node, order):
if node is None:
visitor(node)
else:
op = {
'N': lambda: visitor(node),
'L': lambda: traversal(node.left, order),
'R': lambda: traversal(node.right, order),
}
for x in order:
op[x]()

#traversal the tree level by level
def traversal_level_by_level(node):
if node is not None:
current_level = [node]
while current_level:
next_level = list()
for n in current_level:
if type(n) is str:
output.write('N ')
else:
output.write('%i ' % n.data)
if n.left is not None:
next_level.append(n.left)
else:
next_level.append('N')
if n.right is not None:
next_level.append(n.right)
else:
next_level.append('N ')

output.write('\n')
current_level = next_level

if __name__ == '__main__':
for order in ['NLR', 'LNR', 'LRN']:
if order == 'NLR':
output.write('this is preorder traversal:')
traversal(tree, order)
output.write('\n')
elif order == 'LNR':
output.write('this is inorder traversal:')
traversal(tree, order)
output.write('\n')
else:
output.write('this is postorder traversal:')
traversal(tree, order)
output.write('\n')

output.write('traversal level by level as below:'+'\n')
traversal_level_by_level(tree)

print(output.getvalue())

测试使用的tree截图如下：

csdn博客不能在代码注释中正确显示topo ..

运行结果：

参考：

http://www.laurentluce.com/posts/binary-search-tree-library-in-python/

http://articles.leetcode.com/how-to-pretty-print-binary-tree

https://wizardforcel.gitbooks.io/liaoxuefeng/content/py3/60.html

http://stackoverflow.com/questions/13674772/printing-out-a-binary-search-tree-with-slashes

https://www.reddit.com/r/learnprogramming/comments/14681r/printing_out_a_binary_search_tree_in_level_order/

http://articles.leetcode.com/how-to-pretty-print-binary-tree/