LeetCode 814. Binary Tree Pruning



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]
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]


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


Check the subtree recursively, if the node value is 1, or the node contains subrees with value 1, keep the subtree, otherwise, make the node null.

Python Solution

# 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:
        if not self.helper(root):
            return None
        return root
    def helper(self, root):
        if not root:
            return False
        left = self.helper(root.left)
        right = self.helper(root.right)
        if not left:
            root.left = None
        if not right:
            root.right = None
        return root.val == 1 or left or right
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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