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。