101. 对称二叉树

一、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 isSymmetric(self, root: TreeNode) -> bool:
        def ismirror(left, right):
            if left is None and right is None:
                return True
            if left and right and left.val == right.val:
                return ismirror(left.left, right.right) and ismirror(left.right, right.left)
            else:
                return False
        if root == None:
            return True
        return ismirror(root.left, root.right)

1.官方解释很明确

2.递归的主要思想就是反复调用一个相同的函数，把未知的问题变成你已经解决的问题。本题关键的点在于第三层对应关系 ismirror(left.left, right.right) and ismirror(left.right, right.left)

二、迭代