101. 对称二叉树-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_42396719/article/details/135516658

本文介绍了一个名为`Solution`的类，用于判断给定二叉树是否对称。方法包括递归、迭代和层序遍历，通过比较节点值和结构来确定对称性。

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# 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
    #     #递归
    #     return self.compare(root.left, root.right)
    # def compare(self, left, right):
    #     if not left and not right:
    #         return True
    #     if not left and right:
    #         return False
    #     if left and not right:
    #         return False
    #     if left.val != right.val:
    #         return False
    #     compare1 = self.compare(left.left, right.right)
    #     compare2 = self.compare(left.right, right.left)
    #     return compare1 and compare2
        # #迭代
        # st = []
        # st.append(root.left)
        # st.append(root.right)
        # while st:
        #     rightnode = st.pop()
        #     leftnode = st.pop()
        #     if not leftnode and not rightnode:
        #         continue
        #     if not leftnode or not rightnode or leftnode.val != rightnode.val:
        #         return False
        #     st.append(leftnode.left)
        #     st.append(rightnode.right)
        #     st.append(leftnode.right)
        #     st.append(rightnode.left)
        # return True  
        # 层序
        queue = collections.deque([root])

        while queue:
            level_size = len(queue)
            level_vals = []

            for i in range(level_size):
                node = queue.popleft()
                if node:
                    level_vals.append(node.val)
                    # if node.left:
                    queue.append(node.left)
                    # if node.right:
                    queue.append(node.right)
                else:
                    level_vals.append(None)
            print(level_vals)
            if level_vals != level_vals[::-1]:
                return False
        return True