Leetcode: Majority Element

Question

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.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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Solution

Analysis

refer to here
Moore voting algorithm.
We maintain a current candidate and a counter initialized to 0. As we iterate the array, we look at the current element x:
1. If the counter is 0, we set the current candidate to x and the counter to 1.
2. If the counter is not 0, we increment or decrement the counter based on whether x is the current candidate.

After one pass, the current candidate is the majority element. Runtime complexity = O(n).

Code

class Solution(object):
    def majorityElement(self, nums):
        """ :type nums: List[int] :rtype: int """

        most, count = nums[0], 1
        for ind in range(1,len(nums)):
            if nums[ind]==most:
                count += 1
            else:
                if count==0:
                    most = nums[ind]
                else:
                    count -= 1

        return most