【LeetCode with Python】 Binary Tree Level Order Traversal

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原题页面：https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

题目类型：

难度评价：★

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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}

,

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.

二叉树的层次遍历应用在LeetCode中出现了好几次。一般而言二叉树层次遍历是用队列，但是由于本题要区分出每一层，因此用两个数组分别模拟队列。

class Solution:

    # @param root, a tree node
    # @return a list of lists of integers
    def levelOrder(self, root):
        if None == root:
            return [ ]
        results = [ [root.val] ]
        reflist1 = [root]
        while True:
            reflist2 = [ ]
            result = [ ]
            for i in range(0, len(reflist1)):
                cur = reflist1[i]
                if None != cur.left:
                    reflist2.append(cur.left)
                    result.append(cur.left.val)
                if None != cur.right:
                    reflist2.append(cur.right)
                    result.append(cur.right.val)
            if 0 == len(reflist2):
                break
            results.append(result)
            reflist1 = reflist2
        return results