LeetCode #226 翻转二叉树 树 递归

LeetCode #226 翻转二叉树

题目描述

翻转一棵二叉树。

示例：

输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9

输出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

方法一：递归

每次递归交换左右子树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        # 这里不写可能导致13行nonetype object
        if root is None: return None

        temp = root.left
        root.left = self.invertTree(root.right)
        root.right = self.invertTree(temp)
        
        return root

• 时间复杂度： $O(N)$
• 空间复杂度： $O(N)$

方法二：迭代

类似BFS

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        stack = [root]

        while stack:
            curr = stack.pop()
            if curr:
                curr.left, curr.right = curr.right, curr.left
                stack.append(curr.left)
                stack.append(curr.right)
        return root    

• 时间复杂度： $O(N)$
• 空间复杂度： $O(N)$