This is an O(n^2) solution because it would require passing through each number, and then at each number scanning for a count.

A better way to do this is to take advantage of the property of a majority element that it consists of more than half of the set's elements.

What do I mean by this? Let's take another example, [4, 2, 2, 3, 2].

2 is obviously the majority element in the example from a visual scan, but how can we tip the algorithm off to this? Well, suppose we sorted the array:

Solutions

[2, 2, 2, 3, 4]

Notice that if we hone in on index 2, that we see another 2. This is expected-- if we sort an array of numbers, the majority element will represent at least 50% of the elements, so it will always end up at the Math.floor(nums.length / 2)th index.

Depending on the sorting algorithm, this technique is usually O(n * log n) and requires either O(1) space if the sort is in-place, or O(n) space if not.

Another Method

Yet another way to solve this is to use the speed of a hash map! Hash maps allow access/look-ups in constant time, and its keys are guaranteed to be unique.

Solutions

Thus, we can iterate through [2, 2, 2, 3, 4], and store each unique value as a key, and its attached value being the count. Each time we pass it, we increment its count, and use that information to relay we've got a current majority candidate.

1def majority_element(nums)
2  hash = {}
3  # we keep a local max variable to track what
4  # element is in the majority
5  max, val = 0, nil
6  
7  nums.each do |i|
8    # increment or initialize the num as a key
9    if hash.has_key?(i)
10      hash[i] += 1
11    else
12      hash[i] = 1
13    end
14    
15    if hash[i] > max
16      max = hash[i]
17      val = i
18    end
19  end
20
21  return val
22end
23
24puts majority_element([4, 2, 2, 3, 2])

The time complexity of this solution is O(n) as it requires just one pass through the array, but has a space complexity of O(n) since it needs to maintain an auxiliary hash table.