[LeetCode] 814. Binary Tree Pruning

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

Use postorder traversal to check if sub-tree has "one".  Delete the sub-tree if it does not have "one".

# 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: TreeNode) -> TreeNode:

        def postOrder(node):
            if not node:
                return False
            
            hasOneLeft = postOrder(node.left)
            if not hasOneLeft:
                node.left = None
            
            hasOneRight = postOrder(node.right)
            if not hasOneRight:
                node.right = None
            
            return node.val == 1 or hasOneLeft or hasOneRight
        
        return root if postOrder(root) else None