leetcode-169. Majority Element
难度：easy
Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊
n/2 ⌋ times.
You may assume that the array is non-empty and the majority element always exist in the array.
思路：此题所用的思路跟 136-single number 相同，利用dictionary 的key 和value的对用关系来做。