[8种方法]169 Majority Element

• 分类：分治/sort/位运算/vote/HashMap/Random

方法+时间/空间复杂度

169. Majority Element

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.

Example 1:

Input: [3,2,3]
Output: 3

Example 2:

Input: [2,2,1,1,1,2,2]
Output: 2

代码：

HashTable方法:

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        
        count_table={}
        len_=len(nums)
        for i in range(len_):
            if nums[i] not in count_table:
                count_table[nums[i]]=1
            else:
                count_table[nums[i]]+=1
                
            if count_table[nums[i]]>len_//2:
                return nums[i]
        
        return -1

随机数方法:

from random import randint

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        
        len_=len(nums)
        
        while (1):
            my_num=nums[randint(0,len_-1)]
            count=0
            for n in nums:
                if n==my_num:
                    count+=1
                    if count>len_//2:
                        return my_num
                
        return -1

随机数方法:

from random import randint

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        
        len_=len(nums)
        
        while (1):
            my_num=nums[randint(0,len_-1)]
            count=0
            for n in nums:
                if n==my_num:
                    count+=1
                    if count>len_//2:
                        return my_num
                
        return -1

Boyer-Moore Vote方法:

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        
        len_=len(nums)
        
        my_num=nums[0]
        count=0
        for n in nums:
            if n==my_num:
                count+=1
            else:
                count-=1
                if count==0:
                    my_num=n
                    count=1
                
        return my_num

位运算方法:

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        
        len_=len(nums)
        my_num=0
        
        for i in range(32):
            mask= 1 << i
            count=0
            for n in nums:
                if (n & mask):
                    count+=1
                    
            if count>len_//2:
                if i==31:
                    # if i==31,it means it is a negative number
                    my_num = -((1<<31)-my_num)
                else:
                    my_num|=mask
                
        return my_num

简单sort方法:

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
                
        return sorted(nums)[len(nums)//2]

分治方法:

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return -1
        elif len(nums)==1:
            return nums[0]
        
        len_=len(nums)
        left=self.majorityElement(nums[:len_//2])
        right=self.majorityElement(nums[len_//2:])
        
        if left==right:
            return left
        else:
            count=0
            for n in nums:
                if n==left:
                    count+=1
            if count>len_//2:
                return left
            else:
                return right

---

讨论：

1.这道题咱们的速度图就不发了，毕竟这个题的算法特别多。
2.reference:花花酱
3.方法2BST和方法7部分排序法是C++自带方法，Python没有我就忽略，不实现了