LeetCode: 429. N-ary Tree Level Order Traversal

题目

原题地址：https://leetcode.com/problems/n-ary-tree-level-order-traversal/

Given an n-ary tree, return the level order traversal of its nodes' values.

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Example 1:

image

Input: root = [1,null,3,2,4,null,5,6]
Output: [[1],[3,2,4],[5,6]]

Example 2:

image2

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [[1],[2,3,4,5],[6,7,8,9,10],[11,12,13],[14]]

Constraints:

  • The height of the n-ary tree is less than or equal to 1000
  • The total number of nodes is between [0, 10^4]

解法

广度优先，从上到下一层一层遍历，保存每一层的 value。

这个方法的 Python 代码类似下面这样:

"""
# Definition for a Node.
class Node(object):
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children
"""

class Solution(object):
    def levelOrder(self, root):
        if root is None:
            return []

        values = []
        children = [root]

        while children:
            children_values = []
            new_children = []
            for c in children:
                children_values.append(c.val)
                if c.children:
                    new_children.extend(c.children)

            values.append(children_values)
            children = new_children

        return values

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