Leetcode刷题记录——101. 对称二叉树

思路：常规的先序遍历，其顺序为 根-左-右
我们仿照其顺序，定义一种先序遍历-β方法，其顺序为根-右-左
假设一个树为对称二叉树，那么常规先序遍历方法和先序遍历-β方法的遍历结果应该是相同的
本体采用这种思路，将一棵树的上述两种遍历结果进行对比，若相同，则为对称二叉树

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

class Solution:
    def __init__(self):
        self.leftlist = []
        self.rightlist = []
    def isSymmetric(self, root: TreeNode) -> bool:
        if root is None:
            return True
        elif root.left is None and root.right is None:
            return True
        elif root.left is None or root.right is None:
            return False
        self.root_left_right(root)
        self.root_right_left(root)
        return True if self.leftlist == self.rightlist else False

    def root_left_right(self,node):#常规先序遍历
        if node is None:
            self.leftlist.append('x')
        else:
            self.leftlist.append(node.val)
            self.root_left_right(node.left)
            self.root_left_right(node.right)
    def root_right_left(self,node):#先序-beta
        if node is None:
            self.rightlist.append('x')
        else:
            self.rightlist.append(node.val)
            self.root_right_left(node.right)
            self.root_right_left(node.left)