102-binary-tree-level-order-traversal.py

"""
Problem Link: https://leetcode.com/problems/binary-tree-level-order-traversal/

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]
]
"""
# 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]]
        """
        if root is None:
            return []
        
        current, res = [root], []
        
        while current:
            nex, temp = [], []
            for n in current:
                temp.append(n.val)
                if n.left:
                    nex.append(n.left)
                if n.right:
                    nex.append(n.right)
            current = nex
            res.append(temp)
        return res