LeetCode#169 Majority Element

Problem Definition:

　　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.

Solution 1: 用字典来保存计数，简单粗暴

 

1 def majorityElement(self,nums):
2     d={}
3     for i in nums:
4         d.setdefault(i,0)
5         d[i]+=1
6         if d[i]>len(nums)/2:
7             return i

 

Solution 2:　　1) 排序；　　2) 中间往右一个位置的值即所求;　　3)最快O(nlogn)

　　栗子：[1,2,3,4,4,4,4]　　[1,1,1,1,2,3]

1 def majorityElement(nums):
2         return sorted(nums)[len(nums)/2]

 

Solution 3: 投票，与前一元素相同投1，不同投-1，O(n)

1 def majorityElement(self,nums):
2     major,count,n=0,0,len(nums)
3     for i in nums:
4         if count==0:
5             major=i
6             count=1
7         else:
8             count+=(1 if i==major else -1)
9     return major

 

Solution 4: 不断地随机拿一个数，计数，看运气。

Solution 5: 位运算。(Python 3以前，作位移操作，溢出的int会变成long)

 1 def majorityElement(self,nums):
 2     major,mask,n=0,1,len(nums)
 3     m=32
 4     while m>0:
 5         bc=0
 6         for i in nums:
 7             if i&mask:
 8                 bc+=1
 9             if bc>n/2:
10                 major|=mask
11                 break
12         mask<<=1
13         m-=1
14     return major