【leetcode】1161. Maximum Level Sum of a Binary Tree

题目如下：

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.

Note:

1. The number of nodes in the given tree is between 1 and 10^4.
2. -10^5 <= node.val <= 10^5

解题思路：没什么好说的，依次计算出每一层的值，求出最大即可。

代码如下：

# 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):
    dic = {}
    def recursive(self,node,level):
        self.dic[level] = self.dic.setdefault(level,0) + node.val
        if node.left != None:
            self.recursive(node.left,level+1)
        if node.right != None:
            self.recursive(node.right,level+1)

    def maxLevelSum(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.dic = {}
        self.recursive(root,1)
        max_val = 0
        res = 0
        for key,val in self.dic.iteritems():
            if max_val < val:
                max_val = val
                res = key
        return res