LeetCode: 814. Binary Tree Pruning

题目

原题地址：https://leetcode.com/problems/binary-tree-pruning/

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.)

Example 1:

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.

image1

Example 2:

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

image2

Example 3:

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

image3

Note:

  • The binary tree will have at most 200 nodes.
  • The value of each node will only be 0 or 1.

解法

前序遍历二叉树，在遍历的过程中重建二叉树，将不满足条件的节点删除（节点和子节点的值都不等于 1 ）

这个方法的 Python 代码类似下面这样:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def pruneTree(self, root):
        if root is None:
            return

        # 将不满足条件的节点置为 None
        if not (root.val == 1 or
            (root.left is not None and root.left.val == 1) or
            (root.right is not None and root.right.val == 1)):
            return

        root.left = self.pruneTree(root.left)
        root.right = self.pruneTree(root.right)

        return root

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