leetcode101-python 对称二叉树

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

解题要点：

1.可以把问题分解为第二层的两个子树为对称的，可以近似地利用之前判断两个二叉树是否相同的结论；

# 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):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root == None:
            return True
        else:
            return self.isSym(root.left, root.right)
        
    def isSym(self, p, q):
        if p == None and q == None:
            return True
        elif p == None or q == None:
            return False
        elif p.val == q.val:
            return self.isSym(p.left, q.right) and self.isSym(p.right, q.left)
        else:
            return False