Program.cs

/*public class Solution

/*
 Given two strings s and t, return true if s is a subsequence of t, or false otherwise.
A subsequence of a string is a new string that is formed from the original string by deleting some (can be none)
of the characters without disturbing the relative positions of the remaining characters.
(i.e., "ace" is a subsequence of "abcde" while "aec" is not).
  */
//{
/* public static bool IsSubsequence(string s, string t)
 {
     int tIndex = 0;  // Pointer for string t

     // Loop through every character in string s
     for (int i = 0; i < s.Length; i++)
     {
         // Find the next occurrence of s[i] in t, starting from tIndex
         while (tIndex < t.Length && t[tIndex] != s[i])
         {
             tIndex++;  // Keep moving forward in t
         }

         // If we reached the end of t without finding s[i], return false
         if (tIndex == t.Length)
         {
             return false;
         }

         // Move to the next character in t for subsequent matches
         tIndex++;
     }

     // All characters of s were found in order in t
     return true;
 }


 /*

 public static  bool IsSubsequence(string s, string t)
 {
     if (s.Length > t.Length) return false;

      List <int>  lastIndex = new List<int>() ;
     for (int i = 0; i < s.Length; i++)
     {
         for (int j = 0; j < t.Length; j++)
         {
             if (s[i] == t[j])
             {
                 if (lastIndex.Count > i)  // If an index for this character in s already exists
                 {
                     lastIndex[i] = j;  // Update the index
                 }
                 else
                 {
                     lastIndex.Add(j);  // Add the first occurrence
                 }
             }
             }
         if(lastIndex.Count-1 < i) return false;
     }

     return lastIndex.SequenceEqual(lastIndex.OrderBy(x => x));
 }
 */
/*
 public static void Main()
 {
     Console.WriteLine("\"abc\", \"ahbgdc\"" + IsSubsequence("abc", "ahbgdc")); // Output: true
     Console.WriteLine("\"axc\", \"ahbgdc\"" + IsSubsequence("axc", "ahbgdc")); // Output: false
     Console.WriteLine("\"ace\", \"abcde\""+IsSubsequence("ace", "abcde"));  // Output: true
     Console.WriteLine("\"aec\", \"abcde\"" + IsSubsequence("aec", "abcde"));  // Output: false
     Console.WriteLine("s = \"gh\", t = \"gheg\"" + IsSubsequence( "gh", "gheg"));  // Output: false

 }
}
*/
internal class Program
{
    private static string _GetRange(int first, int last)
    {
        return (Convert.ToInt32(first) != last) ? $"{first}->{last}" : $"{first}";
    }

    public static IList<string> SummaryRanges(int[] nums)
    {
        if (nums == null || nums.Length == 0)
        {
            return [];
        }

        IList<string> result = new List<string>();
        string firstItemInRange = string.Empty;

        for (int i = 0; i < nums.Length; i++)
        {
            if (string.IsNullOrWhiteSpace(firstItemInRange))
            {
                firstItemInRange = $"{nums[i]}";
            }

 

            string range = _GetRange(Convert.ToInt32(firstItemInRange), nums[i]);

            result.Add(range);
            firstItemInRange = string.Empty;
        }

        return result;
    }

    private static void Main(string[] args)
    {
        int[] nums = [-2147483648, -2147483647, 2147483647];

        Console.WriteLine(string.Join(',', SummaryRanges(nums)));

        int[] nums2 = new int[] { 0, 1, 2, 4, 5, 7 };
        Console.WriteLine(string.Join(", ", SummaryRanges(nums2)));

        int[] nums1 = new int[] { 0, 2, 3, 4, 6, 8, 9 };
        Console.WriteLine(string.Join(", ", SummaryRanges(nums1)));

        Console.ReadKey();
    }
}