LeetCode #229: Majority Element II

Problem Statement

(Source) Given an integer array of size n, find all elements that appear more than

⌊ n/3 ⌋

times. The algorithm should run in linear time and in

O(1)

space.

Solution

This problems can be solved using a modified version of the

Boyer–Moore majority vote algorithm

(wiki).

There could be at most 2 elements that appear more than

⌊ n/3 ⌋

times in an given array of size n. Hence the idea is to keep two possible candidates at the same time, and do a “appearance times check” to see if any of the possible candidates has appearance times more than

⌊ n/3 ⌋

after the main loop.