【LeetCode】508. Most Frequent Subtree Sum【M】【40】

Given the root of a tree, you are asked to find the most frequent subtree sum. The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself). So what is the most frequent subtree sum value? If there is a tie, return all the values with the highest frequency in any order.

Examples 1
Input:

  5
 /  \
2   -3

return [2, -3, 4], since all the values happen only once, return all of them in any order.

Examples 2
Input:

  5
 /  \
2   -5

return [2], since 2 happens twice, however -5 only occur once.

Note: You may assume the sum of values in any subtree is in the range of 32-bit signed integer.

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這個用遞歸做還是很舒服的，而且就是標準的遞歸應用場景，對每個節點算sum，然後字典排序就行了

# 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 findFrequentTreeSum(self, root):
        if not root:
            return []
        dic = {}

        def calSum(root):
            if root == None:
                return 0
            s = calSum(root.left) + calSum(root.right) + root.val
            dic[s] = dic.get(s,0) + 1
            return s

        calSum(root)

        l = sorted(dic.iteritems(),key = lambda k:k[1],reverse = True)

        maxx = l[0][1]

        res = []
        for k,v in l:
            if v == maxx:
                res += k,
            else:
                break
        return res
        """
        :type root: TreeNode
        :rtype: List[int]
        """