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0101.py
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0101.py
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# Source: https://leetcode.com/problems/symmetric-tree
# Title: Symmetric Tree
# Difficulty: Easy
# Author: Mu Yang <http://muyang.pro>
################################################################################################################################
# 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.
#
################################################################################################################################
# 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:
"""Iteration"""
def isSymmetric(self, root: TreeNode) -> bool:
if root:
for left, right in zip(self.traversalLeft(root.left), self.traversalRight(root.right)):
if left != right:
return False
return True
def traversalLeft(self, root):
stack = [(None, None,)]
status = None
while stack:
if root:
if status is None: # start
yield root.val
stack.append((root, 'L'))
root = root.left
status = None
elif status == 'L': # Left finished
stack.append((root, 'R'))
root = root.right
status = None
else: # Right finished
root, status = stack.pop()
else:
yield None
root, status = stack.pop()
def traversalRight(self, root):
stack = [(None, None,)]
status = None
while stack:
if root:
if status is None: # start
yield root.val
stack.append((root, 'R'))
root = root.right
status = None
elif status == 'R': # Right finished
stack.append((root, 'L'))
root = root.left
status = None
else: # Right finished
root, status = stack.pop()
else:
yield None
root, status = stack.pop()
################################################################################################################################
class Solution2:
"""Recursion"""
def isSymmetric(self, root: TreeNode) -> bool:
if root:
for left, right in zip(self.traversalLeft(root.left), self.traversalRight(root.right)):
if left != right:
return False
return True
def traversalLeft(self, root):
if root:
yield root.val
yield from self.traversalLeft(root.left)
yield from self.traversalLeft(root.right)
else:
yield None
def traversalRight(self, root):
if root:
yield root.val
yield from self.traversalRight(root.right)
yield from self.traversalRight(root.left)
else:
yield None