题目：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：已知这样的主元素存在，那么根据它的定义可知，这个数字的数量一定比其他数字的数量和大。因此若每次删除两个不相等的数字，那么最后剩下的数字即为所求。
代码：
def majorityElement(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
if nums == None:
return None
num = None
counter = 0
for n in nums:
if counter == 0:
counter = 1
num = n
elif n == num:
counter += 1
else:
counter -= 1
return num
