Find a number where it appears exactly N/2 times

Question

Here is one of my interview question. Given an array of N elements and where an element appears exactly N/2 times and the rest N/2 elements are unique

Answer 1

Here is Don Johe's answer in Ruby:

#!/usr/bin/ruby1.8

def find_repeated_number(a)
  return nil unless a.size >= 3
  (0..a.size - 3).each do |i|
    [
      [0, 1],
      [0, 2],
      [1, 2],
    ].each do |j1, j2|
      return a[i + j1] if a[i + j1] == a[i + j2]
    end
  end
end

p find_repeated_number([1, 1, 2])   # => 1
p find_repeated_number([2, 3, 2])   # => 1
p find_repeated_number([4, 3, 3])   # => 1

O(n)