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

Solution

Solution 1: First attempt

class Solution:
    def majorityElement(self, nums: List[int]) -> int:
        # O(n) time complexity.
        # O(n) space complexity.
        dic = {}
        m = len(nums)//2
        
        for elt in nums:
            if not elt in dic:
                dic[elt] = 1
            else:
                dic[elt] += 1
                if dic[elt] > m:
                    return elt
        return nums[0]

Solution 2: Boyer-Moore Voting Algorithm

class Solution:
    def majorityElement(self, nums: List[int]) -> int:
        # O(n) time complexity.
        # O(1) space complexity.
        count = 0
        candidate = None

        for num in nums:
            if count == 0:
                candidate = num
            count += (1 if num == candidate else -1)

        return candidate