https://leetcode-cn.com/submissions/detail/305476984/
    需要注意的点
    1.迭代
    2.递归

    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, val=0, left=None, right=None):
    4. #         self.val = val
    5. #         self.left = left
    6. #         self.right = right
    7. class Solution:
    8.     def isSymmetric(self, root: Optional[TreeNode]) -> bool:
    9.         if not root:
    10.             return True
    12.         left = root.left
    13.         right = root.right
    15.         if (not left and right) or (not right and left):
    16.             return False
    18.         q = [left, right]
    20.         while q:
    21.             n1 = q.pop(0)
    22.             n2 = q.pop(0)
    24.             if not n1 and not n2:
    25.                 continue
    27.             if not n1 or not n2:
    28.                 return False
    30.             if n1.val != n2.val:
    31.                 return False
    33.             q.append(n1.left)
    34.             q.append(n2.right)
    35.             q.append(n1.right)
    36.             q.append(n2.left)
    38.         return True
    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, val=0, left=None, right=None):
    4. #         self.val = val
    5. #         self.left = left
    6. #         self.right = right
    7. class Solution:
    8.     def isSymmetric(self, root: Optional[TreeNode]) -> bool:
    9.         if not root:
    10.             return True
    12.         def reverseTree(left: TreeNode, right: TreeNode)-> bool:
    13.             if not left or not right:
    14.                 return left == right
    16.             if left.val != right.val:
    17.                 return False
    19.             return (reverseTree(left.left, right.right) and reverseTree(left.right, right.left)) 
    21.         return reverseTree(root.left, root.right)