本文已参与掘金创作者训练营第三期「高产更文」赛道，详情查看：掘力计划｜创作者训练营第三期正在进行，「写」出个人影响力
描述
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]
Explanation:
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]
Note:
The number of nodes in the tree is in the range [1, 200].
Node.val is either 0 or 1.
解析
根据题意，就是给出了一个二叉树 root ，然后让我们进行树的剪枝，对不包含 1 的子树进行移除，返回移除了节点后的树的索引。思路比较简单：
就是使用的 DFS ，判断最底层的节点的左子树是否为空，右子树是否为空，如果左右子树都为空且值为 0 的节点，那么将其变为空。然后向上回溯，上层的节点通过同样的方法也能变为空，知道最后将所有节点从下到上都判断一次，然后返回新树的根节点。
解答
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):
"""
:type root: TreeNode
:rtype: TreeNode
"""
def dfs(root):
if not root:
return None
root.left = dfs(root.left)
root.right = dfs(root.right)
if root.val == 0 and not root.left and not root.right:
return None
return root
return dfs(root)
运行结果
Runtime: 20 ms, faster than 65.79% of Python online submissions for Binary Tree Pruning.
Memory Usage: 13.6 MB, less than 28.57% of Python online submissions for Binary Tree Pruning.
解析
习惯了写函数 dfs ，其实不用写也可以，直接把 pruneTree 当作递归函数使用即可。
解答
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):
"""
:type root: TreeNode
:rtype: TreeNode
"""
if not root:
return None
root.left = self.pruneTree(root.left)
root.right = self.pruneTree(root.right)
if root.val == 0 and not root.left and not root.right:
return None
return root
运行结果
Runtime: 20 ms, faster than 65.79% of Python online submissions for Binary Tree Pruning.
Memory Usage: 13.4 MB, less than 54.52% of Python online submissions for Binary Tree Pruning.
原题链接：leetcode.com/problems/bi…
您的支持是我最大的动力
