0814. Binary Tree Pruning

https://leetcode.com/problems/binary-tree-pruning

Description

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.

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