剑指offer27-二叉树的镜像

这个题目比较简单，思路就是依次遍历二叉树的各个节点并交换左右子树。这里复习一下遍历二叉树的三种思路：递归、栈（先序遍历）、队列（层次遍历）

递归

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

class Solution:
    def mirrorTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return root
        root.left,root.right=self.mirrorTree(root.right),self.mirrorTree(root.left)
        return root

辅助栈

class Solution:
    def mirrorTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return root
        s=[root]
        while len(s):
            t=s.pop()
            if t.left:
                s.append(t.left)
            if t.right:
                s.append(t.right)
            t.left,t.right=t.right,t.left
        return root

队列

class Solution:
    def mirrorTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return root
        s=[root]
        while len(s):
            t=s.pop()
            if t.left:
                s.insert(0,t.left)
            if t.right:
                s.insert(0,t.right)
            t.left,t.right=t.right,t.left
        return root