25期代码随想录算法训练营第十四天 | 二叉树 | 层序遍历（10道题）、226.翻转二叉树 、101.对称二叉树 2

层序遍历 10

链接

# 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 levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:if not root:return []res = []q = deque([root])while q:level = []for _ in range(len(q)):node = q.popleft()if node.left:q.append(node.left)if node.right:q.append(node.right)level.append(node.val)res.append(level)return res

226.翻转二叉树

链接

# 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 invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:if not root:return rootstack = [root]while stack:node = stack.pop()node.left, node.right = node.right, node.leftif node.left:stack.append(node.left)if node.right:stack.append(node.right)return root

101.对称二叉树 2

链接

# 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:def compare(left,right):if left == None and right != None: return Falseelif left == None and right == None: return Trueelif left != None and right == None: return Falseelif left.val != right.val: return Falseoutside = compare(left.left, right.right)inside = compare(left.right,right.left)return outside and insidereturn compare(root.left, root.right)