LeetCode #814 Binary Tree Pruning

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Problem

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.)

Examples

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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.

img

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

img

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

img

Solution

Method: Depth-first Search

Time Complexity: O(n)

Space Complexity:

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class Solution:
    def pruneTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        stack = [(root, False)]
        
        while stack:
            node, visited = stack.pop()
            if node:
                if not visited:
                    stack.append((node, True))
                    stack.append((node.right, False))
                    stack.append((node.left, False))
                else:
                    if node.left and node.left.val == -1:
                        node.left = None
                    if node.right and node.right.val == -1:
                        node.right = None
                    if not node.left and not node.right:
                        if node.val == 0:
                            node.val = -1

        return root