Given an array of distinct integer values, count the number of pairs of integers that have difference k.

CountPairsWithDifference.cs

// Given an array of distinct integer values, count the number of pairs of integers that 
// have difference k. For example, given the array [1, 7, 5, 9, 2, 12, 3] and the difference
// k = 2, there are four pairs with difference 2: (1, 3), (3, 5), (5, 7), (7, 9).

CountPairsWithDifference([1, 7, 5, 9, 2, 12, 3], 2).Dump(); // 4
CountPairsWithDifference([int.MaxValue, int.MinValue, int.MaxValue - 2, int.MinValue + 2], 2).Dump(); // 2
CountPairsWithDifference([1, 7, 5, 9, 2, 12, 3], 0).Dump(); // 0
CountPairsWithDifference([1, 7, 5, 9, 2, 12, 3], -2).Dump(); // 4

int CountPairsWithDifference(int[] integers, int diff)
{
    if (diff is 0)
        return 0;
        
    diff = Math.Abs(diff);

    var set = new HashSet<int>(integers);
    var numOfPairs = 0;
    foreach (var i in integers)
        if (i <= int.MaxValue - diff && set.Contains(i + diff))
            numOfPairs++;

    return numOfPairs;
}