leetcode1161 Maximum Level Sum of a Binary Tree

 """
 BFS遍历题，一遍AC
 Given the root of a binary tree, the level of its root is 1, the level of its children is 2, and so on.
 Return the smallest level X such that the sum of all the values of nodes at level X is maximal.
 Example 1:
 Input: [1,7,0,7,-8,null,null]
 Output: 2
 Explanation:
 Level 1 sum = 1.
 Level 2 sum = 7 + 0 = 7.
 Level 3 sum = 7 + -8 = -1.
 So we return the level with the maximum sum which is level 2.
 """
 class TreeNode:
     def __init__(self, x):
         self.val = x
         self.left = None
         self.right = None

 class Solution:
     def maxLevelSum(self, root):
         if root == None:
             return 0
         queue = []
         res = []
         queue.append(root)
         while queue:
             n = len(queue)
             sum = 0
             newqueue = []
             for _ in range(n):
                 x = queue.pop(0)
                 sum += x.val
                 if x.left != None:
                     newqueue.append(x.left)
                 if x.right != None:
                     newqueue.append(x.right)
             queue = newqueue
             res.append(sum)
         return res.index(max(res))+1