814. Binary Tree Pruning

https://leetcode.com/problems/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.

https://s3-lc-upload.s3.amazonaws.com/uploads/2018/04/06/1028_2.png

Example 2:

Input: [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

https://s3-lc-upload.s3.amazonaws.com/uploads/2018/04/06/1028_1.png

Example 3:

Input: [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]

https://s3-lc-upload.s3.amazonaws.com/uploads/2018/04/05/1028.png

Note:

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