102. 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:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        list = []
        result = []
        if not root:
            return result
        list.append(root)
        while len(list):
            temp = []
            for i in range(len(list)):
                current = list.pop(0)
                temp.append(current.val)
                if current.left:
                   list.append(current.left)
                if current.right:
                 list.append(current.right)
            result.append(temp)
        return result