FindParityOutliers

Given a collection, returns the items that are parity outliers.

• Use IEnumerable.GroupBy() to create groups for each parity (0 and 1).
• Use IEnumerable.OrderBy() in combination with IEnumerable.Count() to order the two groups in ascending order based on frequency.
• Use IEnumerable.First() to get the first element and return its Key property, which corresponds to the least common parity value.
• Finally, use IEnumerable.Where() to get all elements with the least common parity value.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static IEnumerable<int> FindParityOutliers(IEnumerable<int> items)
  {
    return items.Where(
      i => i % 2 == items
        .GroupBy(i => i % 2)
        .OrderBy(i => i.Count())
        .First()
        .Key
    );
  }
}

int[] nums = {1, 2, 3, 4, 6};

_30s.FindParityOutliers(nums); // {1, 3}