Given the `root` of a binary tree, return the same tree where every subtree (of the given tree) not containing a `1` has been removed.

A subtree of a node `node` is `node` plus every node that is a descendant of `node`.

Example 1:

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

Example 3:

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

Constraints:

- The number of nodes in the tree is in the range `[1, 200]`.
- `Node.val` is either `0` or `1`.