leetcode 229[medium]--- Majority Element II
难度：medium
Given an integer array of size n,
find all elements that appear more than ⌊ n/3 ⌋ times.
The algorithm should run in linear time and in O(1) space.
思路：找到给定的array里面，出现次数超过总长度 1/3 的element。用hashtable解决，有点不明白为什么是medium难度。