LeetCode 102. Binary Tree Level Order Traversal--递归，迭代-Python，Java解法

题目地址：

---

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

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这道题可以用迭代做，也可以用递归做。
Python递归解法（比较简单）：

class Solution:
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        levels = []
        if not root:
            return levels
        
        def helper(node, level):
            # start the current level
            if len(levels) == level:
                levels.append([])

            # append the current node value
            levels[level].append(node.val)

            # process child nodes for the next level
            if node.left:
                helper(node.left, level + 1)
            if node.right:
                helper(node.right, level + 1)
            
        helper(root, 0)
        return levels

Python迭代解法,使用队列

from collections import