Yandex - Intervals.cs

// [1,4,5,2,3,9,8,11,0] => "0-5,8-9,11" // [0,1,2,3,4,5,8,9,11]
// [1,4,3,2] => "1-4"                   // [1,2,3,4]
// [1,4] => "1,4"
//
// [1, 2, 3, 4, 5], 7]

private static string ParseToInterval(int[] nums)
{
    if (nums.Length == 0)
    {
        return string.Empty;
    }

    Arrays.Sort(nums);

    var start = 0;
    var prev = 0;
    var current = 0;

    var strList = new List<string>();

    while(current < nums.Length)
    {
        current++;

        if (nums[current] - nums[prev] == 1) {
            prev++;
        }
        else
        {
            strList.Add(start == prev ? nums[start].ToString() : $"{nums[start]}-{nums[prev]}");
            start = current;
            prev = current;
        }
    }

    strList.Add(start == prev ? nums[start].ToString() : $"{nums[start]}-{nums[prev]}");

    return string.Join(", ", strList);
}