leetcode814 Binary Tree Pruning

 1 """
 2 We are given the head node root of a binary tree, where additionally every node‘s value is either a 0 or a 1.
 3 Return the same tree where every subtree (of the given tree) not containing a 1 has been removed.
 4 (Recall that the subtree of a node X is X, plus every node that is a descendant of X.)
 5 Example 1:
 6 Input: [1,null,0,0,1]
 7 Output: [1,null,0,null,1]
 8 Explanation:
 9 Only the red nodes satisfy the property "every subtree not containing a 1".
10 The diagram on the right represents the answer.
11 Example 2:
12 Input: [1,0,1,0,0,0,1]
13 Output: [1,null,1,null,1]
14 Example 3:
15 Input: [1,1,0,1,1,0,1,0]
16 Output: [1,1,0,1,1,null,1]
17 """
18 """
19 剪枝问题：
20 递归遍历，不用考虑递归左右顺序
21 如果结点值为0且，左右孩子为None
22 将该结点置为None
23 """
24 class TreeNode:
25     def __init__(self, x):
26         self.val = x
27         self.left = None
28         self.right = None
29 
30 class Solution:
31     def pruneTree(self, root):
32         if root == None:
33             return None
34         root.left = self.pruneTree(root.left)
35         root.right = self.pruneTree(root.right)
36         if root.val == 0 and root.left == None and root.right == None: #！！！判别式
37             return None
38         return root