思路：递归的思想，当头节点不存在时直接返回True，递归遍历左右节点的值。

Python：

# 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 compare(root1,root2):
            if(not root1)and(not root2):
                return True
            if(not root1 or not root2)or(root1.val!=root2.val):
                return False
            return compare(root1.left,root2.right) and compare(root1.right,root2.left)
        return compare(root.left,root.right) if root else True