leetcode814 Binary Tree Pruning

"""
We are given the head node root of a binary tree, where additionally every node's value is either a 0 or a 1.
Return the same tree where every subtree (of the given tree) not containing a 1 has been removed.
(Recall that the subtree of a node X is X, plus every node that is a descendant of X.)
Example 1:
Input: [1,null,0,0,1]
Output: [1,null,0,null,1]
Explanation:
Only the red nodes satisfy the property "every subtree not containing a 1".
The diagram on the right represents the answer.
Example 2:
Input: [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]
Example 3:
Input: [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]
"""
"""
剪枝问题：
递归遍历，不用考虑递归左右顺序
如果结点值为0且，左右孩子为None
将该结点置为None
"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def pruneTree(self, root):
        if root == None:
            return None
        root.left = self.pruneTree(root.left)
        root.right = self.pruneTree(root.right)
        if root.val == 0 and root.left == None and root.right == None: #！！！判别式
            return None
        return root