二叉树
组成：
1、根节点 BinaryTree：root
2、每一个节点，都有左子节点和右子节点（可以为空） TreeNode：value、left、right
二叉树的遍历：
遍历二叉树：深度优先遍历、广度优先遍历。
广度：先遍历兄弟节点，再遍历子节点
深度：先遍历子节点，再遍历兄弟节点
上图深度遍历结果：50/20/60/15/30/70
上图广度遍历结果：50/20/15/30/60/70
深度遍历又分为先序、中序、后序的遍历方式：
先序遍历：先根节点，再左子树，再右子树
上图先序遍历结果：50/20/ 15/ 30/ 60/ 70
中序遍历：先左子树，再根节点，再右子树
上图中序遍历结果：15/20/30/50/60/70
后序遍历：先左子树，再右子树，再根节点
上图后序遍历结果：15/30/20/70/60/50
代码实现：
# encoding=utf-8
class TreeNode(object): #定义二叉树类
def __init__(self,val,left=None,right=None):
self.val = val
self.left = left
self.right = right
class BinaryTree(object):
def __init__(self,root=None):
self.root = root
def preScan(self,retList, node): #先序遍历：先跟、再左、后右
if node != None:
retList.append(node.val)
self.preScan(retList, node.left)
self.preScan(retList, node.right)
return retList
def midScan(self, retList, node): #中序遍历：先左、再跟、后右
if node != None:
self.midScan(retList, node.left)
retList.append(node.val)
self.midScan(retList, node.right)
return retList
def postScan(self, retList, node): #后序遍历：先左、再右、后跟
if node != None:
self.postScan(retList, node.left)
self.postScan(retList, node.right)
retList.append(node.val)
return retList
if __name__ =='__main__':
root = TreeNode(50)
root.left = TreeNode(20,left=TreeNode(15),right=TreeNode(30,right=TreeNode(12)))
root.right = TreeNode(60,right=TreeNode(70))
bTree = BinaryTree(root)
retList = bTree.preScan([],bTree.root)
print retList
retList2 = bTree.midScan([],bTree.root)
print retList2
retList3 = bTree.postScan([],bTree.root)
print retList3