二叉树三大基础知识 Python专题

def count(n):
    s=count_searched.get(n,0)
    if not s:
        for k in range(0,n):
            s+=count(k)*count(n-1-k)
        count_searched[n]=s
    return s
count_searched={0:1}

要创建以上的二叉树 我们需要定义TreeNode（）这样一个类
class Treenode:
    def __init__(self,data,left=None,right=None):
        self.data=data
        self.left=left
        self.right=right
    def __str__(self):
        return str(self.data)
A,B,C,D,E,F,G,H,I=[Treenode(x) for x in 'ABCDEFGHI']#列表拆包
A.left,A.right=B,C
B.right=D
C.left,C.right=E,F
E.left=G
F.left,F.rihgt=H,I
print(A.left,B.right,F.left)#检验

def pre_order(root):#前序遍历 先根节点然后左子树然后右子树
    print(root)
    if root.left:pre_order(root.left)
    if root.right:pre_order(root.right)

def in_order(root):#先打印左子树再根节点再右子树
    if root.left:in_order(root.left)
    print(root)
    if root.right:in_order(root.right)

def post_order(root):
    if root.left:post_order(root.left)
    if root.right:post_order(root.right)
    print(root)#先打印左子树再打印右子树再打印根节点