101.symmetric-tree.py

#
# @lc app=leetcode id=101 lang=python3
#
# [101] Symmetric Tree
#
# https://leetcode.com/problems/symmetric-tree/description/
#
# algorithms
# Easy (53.34%)
# Likes:    12112
# Dislikes: 274
# Total Accepted:    1.5M
# Total Submissions: 2.8M
# Testcase Example:  '[1,2,2,3,4,4,3]'
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# Given the root of a binary tree, check whether it is a mirror of itself
# (i.e., symmetric around its center).
# 
# 
# Example 1:
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# Input: root = [1,2,2,3,4,4,3]
# Output: true
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# 
# Example 2:
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# Input: root = [1,2,2,null,3,null,3]
# Output: false
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# 
# 
# Constraints:
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# The number of nodes in the tree is in the range [1, 1000].
# -100 <= Node.val <= 100
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# 
# 
# Follow up: Could you solve it both recursively and iteratively?
#

# @lc code=start
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True
        return self.isSymmetricHelper(root.left, root.right)
    
    def isSymmetricHelper(self, left: Optional[TreeNode], right: Optional[TreeNode]) -> bool:
        if not left and not right:
            return True
        if not left or not right:
            return False
        if left.val != right.val:
            return False
        return self.isSymmetricHelper(left.left, right.right) and self.isSymmetricHelper(left.right, right.left)
        
# @lc code=end