用两个指针去遍历这棵树，（并使用深度优先 中序遍历方法）一个指针从左方向开始遍历，一个指针从右方向开始遍历。比较结构与数据

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def isbalance(self, p: Optional[TreeNode], q: Optional[TreeNode]):
        if p is None and q is None:
            return True
        elif p is None or q is None:
            return False
        elif p.val != q.val:
            return False
        else:
            return self.isbalance(p.left,q.right) and self.isbalance(p.right,q.left)
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        return self.isbalance(root,root)