101 Symmetric Tree

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

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• 要求： 给一个binary tree, 判断它是不是对称

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• 返回：
  True/False

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• 例子：

  • input: [1,2,2,3,4,4,3]
    1 /
    2 2 / \ /
    3 4 4 3

  • output: True

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Solution

判断根的left & right children是否相等;
如果相等，判断left.right & right.left, 以及left.left & right.right 是否相等，do this recursively.

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

class Solution(object):
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root:
            return self.help(root.left, root.right)
        return True

    def help(self, p, q):
        if p == None and q == None: return True
        if p and q and p.val == q.val:
            return self.help(p.right, q.left) and self.help(p.left, q.right)
        else:
            return False