## Description

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]


Note:

• The binary tree will have at most 100 nodes.
• The value of each node will only be 0 or 1.

## Solutions

### 1. Recursion

# Time: O(logn)
# Space: O(1)
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
def pruneTree(self, root: TreeNode) -> TreeNode:
if not root:
return None
if root.left:
root.left = self.pruneTree(root.left)
if root.right:
root.right = self.pruneTree(root.right)
if not root.left and not root.right and root.val == 0:
return None
return root
# 28/28 cases passed (28 ms)
# Your runtime beats 71.93 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.6 MB)