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