Leetcode 532 - pass leetcode 532 online judge

Leetcode532 K-diff pairs in an array.cs

public class Solution {
    public int FindPairs(int[] arr, int k)
        {
            // your code goes hereif
            if (arr == null || arr.Length == 0 || k < 0)
            {
                return 0;
            }

            // assume that arr is not empty
            var pairs = new List<int[]>(); // int[0] -> bigger value
            var set = new HashSet<int>();
            var existing = new HashSet<int>(); 

            foreach (var number in arr) // 0, -1, -2 , 2, 1
            {
                var searchBigger = number + k; // 1 , 0, -1, 3, 2
                var searchSmaller = number - k; // -1, -2, -3, 1, 0 

                if (set.Contains(searchBigger)) // true, 2
                {
                    // add the pair
                    if (!existing.Contains(searchBigger))
                    {
                        pairs.Add(new int[] { searchBigger, number }); // (0, -1), (-1, -2), [2, 1]
                        existing.Add(searchBigger); 
                    }
                }

                if (set.Contains(searchSmaller)) // -1=> true, 
                {
                    // add the pair
                    if (!existing.Contains(number))
                    {
                        pairs.Add(new int[] { number, searchSmaller }); // -> [1, 0]
                        existing.Add(number); 
                    }
                }

                set.Add(number);   // {0, -1, -2, 2}
            }

            return pairs.Count; // 
        }
}