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LC814 - Binary Tree Prunting

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Datastructure and Algorithm: DAY 51. Leetcode: 205/2710

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:

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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:

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

Example 3:

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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.

Solutions

This is a post-order traversal

If the following conditions are met, delete the current leaf node

  1. Left subtree is None
  2. The right subtree is None
  3. The current root value is 0
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# O(n) time | O(n) space
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def pruneTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        if root is None:
            return
        
        root.left = self.pruneTree(root.left)
        root.right = self.pruneTree(root.right)

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