LeetCode: 101. Symmetric Tree

题目

原题地址:https://leetcode.com/problems/symmetric-tree/

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

Follow up: Solve it both recursively and iteratively.

解法

从上到下一层一层进行比较。

这个方法的 Python 代码类似下面这样:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def isSymmetric(self, root):
        if root is None:
            return True
        return self.is_mirror(root.left, root.right)

    def is_mirror(self, left, right):
        if left is None and right is None:
            return True
        if left is None or right is None:
            return False
        if left.val != right.val:
            return False
        return self.is_mirror(left.left, right.right) and self.is_mirror(left.right, right.left)

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