题目描述：

LeetCode 501. Find Mode in Binary Tree

Given a binary tree with duplicates. You have to find all the mode(s) in given binary tree.

For example:
Given binary tree [1,null,2,2],

   1
    \
     2
    /
   2

return [2].

Note: If a tree has more than one mode, you can return them in any order.

题目大意：

给定一棵包含重复元素的二叉树。寻找其中的所有众数。

注意：二叉树可能包含多个众数，以任意顺序返回即可。

解题思路：

二叉树遍历 + 计数

Python代码：

# 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 findMode(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        counter = collections.Counter()
        def traverse(root):
            if not root: return
            counter[root.val] += 1
            traverse(root.left)
            traverse(root.right)
        traverse(root)
        maxn = max(counter.values() + [None])
        return [e for e, v in counter.iteritems() if v == maxn]

 

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