C# Program To Remove Duplicates From A Given String - GeeksforGeeks

11 Dec, 2021
Given a string S, the task is to remove all the duplicates in the given string. Below are the different methods to remove duplicates in a string.
METHOD 1 (Simple) 
C#
// C# program to remove duplicate character// from character array and print in sorted// orderusing System;using System.Collections.Generic;class GFG {static String removeDuplicate(char []str, int n){    // Used as index in the modified string    int index = 0;      // Traverse through all characters    for (int i = 0; i < n; i++)    {          // Check if str[i] is present before it         int j;        for (j = 0; j < i; j++)         {            if (str[i] == str[j])            {                break;            }        }          // If not present, then add it to        // result.        if (j == i)         {            str[index++] = str[i];        }    }    char [] ans = new char[index];    Array.Copy(str, ans, index);    return String.Join("", ans);}  // Driver codepublic static void Main(String[] args){    char []str = "geeksforgeeks".ToCharArray();    int n = str.Length;    Console.WriteLine(removeDuplicate(str, n));}}  // This code is contributed by PrinciRaj1992 
Output:  
geksfor
Time Complexity : O(n * n) Auxiliary Space : O(1) Keeps order of elements the same as input. 
METHOD 2 (Use BST) use set which implements a self-balancing Binary Search Tree. 
C#
// C# program to remove duplicate character// from character array and print in sorted// orderusing System;using System.Collections.Generic;    public class GFG{  static char []removeDuplicate(char []str, int n){          // Create a set using String characters    // excluding '�'    HashSet<char>s = new HashSet<char>(n - 1);    foreach(char x in str)        s.Add(x);              char[] st = new char[s.Count];          // Print content of the set    int i = 0;    foreach(char x in s)       st[i++] = x;        return st;}  // Driver codepublic static void Main(String[] args){   char []str= "geeksforgeeks".ToCharArray();   int n = str.Length;        Console.Write(removeDuplicate(str, n));}}   // This code contributed by gauravrajput1
Output:  
  efgkors
Time Complexity: O(n Log n) Auxiliary Space: O(n)
Thanks to Anivesh Tiwari for suggesting this approach.
It does not keep the order of elements the same as the input but prints them in sorted order.
METHOD 3 (Use Sorting) Algorithm: 
  1) Sort the elements.
  2) Now in a loop, remove duplicates by comparing the 
      current character with previous character.
  3)  Remove extra characters at the end of the resultant string.
Example:  
Input string:  geeksforgeeks
1) Sort the characters
   eeeefggkkorss
2) Remove duplicates
    efgkorskkorss
3) Remove extra characters
     efgkors
Note that, this method doesn’t keep the original order of the input string. For example, if we are to remove duplicates for geeksforgeeks and keep the order of characters the same, then the output should be geksfor, but the above function returns efgkos. We can modify this method by storing the original order.
Implementation:  
C#
// C# program to remove duplicates, the order of// characters is not maintained in this programusing System;      class GFG {    /* Method to remove duplicates in a sorted array */    static String removeDupsSorted(String str)    {        int res_ind = 1, ip_ind = 1;                  // Character array for removal of duplicate characters        char []arr = str.ToCharArray();                  /* In place removal of duplicate characters*/        while (ip_ind != arr.Length)        {            if(arr[ip_ind] != arr[ip_ind-1])            {                arr[res_ind] = arr[ip_ind];                res_ind++;            }            ip_ind++;                      }              str = new String(arr);        return str.Substring(0,res_ind);    }          /* Method removes duplicate characters from the string    This function work in-place and fills null characters    in the extra space left */    static String removeDups(String str)    {    // Sort the character array    char []temp = str.ToCharArray();    Array.Sort(temp);    str = String.Join("",temp);              // Remove duplicates from sorted    return removeDupsSorted(str);    }          // Driver Method    public static void Main(String[] args)    {        String str = "geeksforgeeks";        Console.WriteLine(removeDups(str));    }}  // This code is contributed by 29AjayKumar
Output:  
efgkors
Time Complexity: O(n log n) If we use some nlogn sorting algorithm instead of quicksort.
Auxiliary Space: O(1)
METHOD 4 (Use Hashing ) 
Algorithm:  
1: Initialize:
    str  =  "test string" /* input string */
    ip_ind =  0          /* index to  keep track of location of next
                             character in input string */
    res_ind  =  0         /* index to  keep track of location of
                            next character in the resultant string */
    bin_hash[0..255] = {0,0, ....} /* Binary hash to see if character is 
                                        already processed or not */
2: Do following for each character *(str + ip_ind) in input string:
              (a) if bin_hash is not set for *(str + ip_ind) then
                   // if program sees the character *(str + ip_ind) first time
                         (i)  Set bin_hash for *(str + ip_ind)
                         (ii)  Move *(str  + ip_ind) to the resultant string.
                              This is done in-place.
                         (iii) res_ind++
              (b) ip_ind++
  /* String obtained after this step is "te stringing" */
3: Remove extra characters at the end of the resultant string.
  /*  String obtained after this step is "te string" */
Implementation:  
C#
// C# program to remove duplicatesusing System;using System.Collections.Generic;  class GFG{    /* Function removes duplicate characters     from the string. This function work in-place */    void removeDuplicates(String str)    {        HashSet<char> lhs = new HashSet<char>();        for(int i = 0; i < str.Length; i++)            lhs.Add(str[i]);                  // print string after deleting         // duplicate elements        foreach(char ch in lhs)            Console.Write(ch);    }          // Driver Code    public static void Main(String []args)    {        String str = "geeksforgeeks";        GFG r = new GFG();        r.removeDuplicates(str);    }}  // This code is contributed by Rajput-Ji
Output:  
geksfor
Time Complexity: O(n)
Important Points:  
Method 2 doesn’t maintain the characters as original strings, but method 4 does.
It is assumed that the number of possible characters in the input string is 256. NO_OF_CHARS should be changed accordingly.
calloc() is used instead of malloc() for memory allocations of a counting array (count) to initialize allocated memory to ‘�’. the malloc() followed by memset() could also be used.
The above algorithm also works for integer array inputs if the range of the integers in the array is given. An example problem is to find the maximum occurring number in an input array given that the input array contains integers only between 1000 to 1100
Method 5 (Using IndexOf() method) : Prerequisite : Java IndexOf() method  
C#
// C# program to create a unique stringusing System;       public class IndexOf {          // Function to make the string unique    public static String unique(String s)    {        String str = "";        int len = s.Length;                  // loop to traverse the string and        // check for repeating chars using        // IndexOf() method in Java        for (int i = 0; i < len; i++)         {            // character at i'th index of s            char c = s[i];                          // if c is present in str, it returns            // the index of c, else it returns -1            if (str.IndexOf(c) < 0)            {                // adding c to str if -1 is returned                str += c;            }        }                  return str;    }      // Driver code    public static void Main(String[] args)    {        // Input string with repeating chars        String s = "geeksforgeeks";                  Console.WriteLine(unique(s));    }}  // This code is contributed by Princi Singh
Output:  
geksfor
Thanks debjitdbb for suggesting this approach. 
Method 6 (Using unordered_map STL method) : Prerequisite : unordered_map STL C++ method  
C#
// C# program to create a unique String using unordered_map  /* access time in unordered_map on is O(1) generally if no collisions occur and therefore it helps us check if an element exists in a String in O(1) time complexity with constant space. */using System;using System.Collections.Generic;  public class GFG{ static char[] removeDuplicates(char []s,int n){  Dictionary<char,int> exists = new Dictionary<char, int>();    String st = "";  for(int i = 0; i < n; i++){    if(!exists.ContainsKey(s[i]))    {      st += s[i];      exists.Add(s[i], 1);    }  }  return st.ToCharArray();}  // driver codepublic static void Main(String[] args){  char []s = "geeksforgeeks".ToCharArray();  int n = s.Length;  Console.Write(removeDuplicates(s,n));}}  // This code is contributed by umadevi9616
Output:  
geksfor
Time Complexity : O(n) Auxiliary Space : O(n)Thanks, Allen James Vinoy for suggesting this approach. 
Please refer complete article on Remove duplicates from a given string for more details!
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