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.


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]

class solution:
    def prune(self,root):
        if root==None:
            return None
        root.left=self.prune(root.left)
        root.right=self.prune(root.right)

        if root.val==0 and root.left==None and root.right==None:
            root=None
        return root
