递归方法, 判断左右子树是否对称: 1)左右子树的根节点都为空或者val相同, 2)左子树的左子树与右子树的右子树对称, 3)左子树的右子树与右子树的左子树对称。

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isSymmetric(self, root):
        if root == None: return True
        return self.checkSym(root.left, root.right)
        
    def checkSym(self, left, right):
        if left == right == None: return True
        if not (left and right): return False
        if left.val != right.val: return False
        return self.checkSym(left.left, right.right) and self.checkSym(left.right, right.left)