LeetCode----Binary Tree Level Order Traversal

最新推荐文章于 2022-03-02 11:43:51 发布

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#二叉树 #层次遍历 #LeetCode #python

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本文介绍了一种解决二叉树层次遍历问题的方法。通过使用递归方式，记录每个节点的层级信息，并最终返回按层级排列的节点值列表。

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Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

分析：

二叉树的层次遍历，可以用队列，也可以用队列。

代码：

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        treedict = {}
        self.traverse(root, 0, treedict)
        levelNodes = []
        for k in treedict.keys():
            levelNodes.append(treedict[k])
        return levelNodes

    def traverse(self, root, level, treedict):
        if root:
            if level in treedict:
                treedict[level].append(root.val)
            else:
                treedict[level] = [root.val]
            self.traverse(root.left, level + 1, treedict)
            self.traverse(root.right, level + 1, treedict)