Does the string have unique characters?

Problem

We need an algorithm to determine if a string has all unique characters, none alphabet letters are exempted.

Solution

We will make three methods; the first shall return the alphabet letters; the second will fill an integer array with the frequency of occurrence of each letter; and the last method shall test for uniqueness of the string input.

/*********************************************************
       * Method FillArray shall fill the unique array with the *
       * with the frequency of occurence of each letter in the *
       * string cin                                            *
       * *******************************************************/
      private static void FillArray(char[] cin, int[] chars, int[] unique)
      {
         for (int j = 0; j < cin.Length; j++)
         {
            for (int k = 0; k < unique.Length; k++)
               if (cin[j] == chars[k])
                  unique[k]++;
         }
      }
 
      /***********************************************
       * Method Get Alphabet will return an int array*
       * with all the lower case alphabet letters    *
       * *********************************************/
      private static int[] GetAlphabet(int[] chars, int a)
      {
         for (int i = 0; i < chars.Length; i++)
            chars[i] = a++;
         return chars;
      }
 
      /********************************************
       * The method isUnique shall test the unique*
       * int[] for occurences, if any of the      *
       * elements is bigger than 1, then the      *
       * string is not unique, and hence return   *
       * false                                    *
       * ******************************************/
      static bool isUnique(int[] u)
      {
         for (int i = 0; i < u.Length; i++)
            if (u[i] > 1) return false;
         return true;
      }

A sample input:

string inp = "abcd e f m".ToLower(); //input string
         char[] cin = inp.ToCharArray(); //string to array
         int[] chars = new int[26]; //alphabet letters
         int[] unique = new int[26]; //occurence array
         int a = 97; //the letter a is '97' in ascii
 
         chars = GetAlphabet(chars, a);
         FillArray(cin, chars, unique);
         Console.WriteLine("The string\"{0}\" is unique? {1}", inp, isUnique(unique));

A sample output:



***

Can it be done with no extra data structures in place? Obviously:

private static bool IsStringUnique(char[] t)
      {
         int counter = 0;
         for (int i = 0; i < t.Length; i++)
         {
            for (int j = 0; j < t.Length; j++)
            {
               if (t[i] == t[j])
                  counter++;
            }
            if (counter > 1)
               return false;
            counter = 0;
         }
         return true;
      }

Test input:

char[] t = "The quick brown fox".ToLower().Replace(" ", "").ToCharArray();
         bool b = IsStringUnique(t);
         Console.WriteLine("string is unique? {0}", b);

A sample output:

Compressing a string input

Problem

Compressing a string using the count of repeated characters.  For example, the  string  aabcccccaaa would  become a2blc5a3. If the "compressed" string would  not become smaller than the  original string then we output the original

Solution

private static string CompressString(string a)
      {
         int c = 0; //counter
         int i = 0; //iterator
         char h; //holder
         string cmp = string.Empty; //compressed string
         while (i < a.Length) 
         {
            h = a[i]; 
            cmp += h; //Concat char to string
            while (i != a.Length && h == a[i]) //loop while char is same
            {
               ++c; i++; //increment counter, increment iterator
            }
            cmp += c.ToString(); //concat counter to string
            c = 0; //reinitialize the counter
         }
         if(a.Length>=cmp.Length)
            return cmp;
         return a;
      }

As a test case:

string a = "aaabbsssdddccddeaassssssse".ToLower();
         string cmp = CompressString(a);
         Console.WriteLine(cmp);

This should output a compressed string: