Remove duplicates from a given string

Difficulty Level : Easy
Last Updated : 24 Jul, 2020

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++

// CPP program to remove duplicate character 
// from character array and print in sorted 
// order 
#include <bits/stdc++.h> 
using namespace std; 
  
char *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]; 
   } 
     
   return str; 
} 
  
// Driver code 
int main() 
{ 
   char str[]= "geeksforgeeks"; 
   int n = sizeof(str) / sizeof(str[0]); 
   cout << removeDuplicate(str, n); 
   return 0; 
} 

Java

// Java program to remove duplicate character 
// from character array and print in sorted 
// order 
import java.util.*; 
  
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]; 
            } 
        } 
        return String.valueOf(Arrays.copyOf(str, index)); 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        char str[] = "geeksforgeeks".toCharArray(); 
        int n = str.length; 
        System.out.println(removeDuplicate(str, n)); 
    } 
} 
  
// This code is contributed by Rajput-Ji 

Python3

# Python3 program to remove duplicate character 
# from character array and prin sorted 
# order 
def removeDuplicate(str, n): 
      
    # Used as index in the modified string 
    index = 0
      
    # Traverse through all characters 
    for i in range(0, n): 
          
        # Check if str[i] is present before it  
        for j in range(0, i + 1): 
            if (str[i] == str[j]): 
                break
                  
        # If not present, then add it to 
        # result. 
        if (j == i): 
            str[index] = str[i] 
            index += 1
              
    return "".join(str[:index]) 
  
# Driver code 
str= "geeksforgeeks"
n = len(str) 
print(removeDuplicate(list(str), n)) 
  
# This code is contributed by SHUBHAMSINGH10 

C#

// C# program to remove duplicate character 
// from character array and print in sorted 
// order 
using 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 code 
public 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 same as input.

METHOD 2 (Use BST)
use set in c++ stl which implements a self balancing Binary Search Tree.

// CPP program to remove duplicate character 
// from character array and print in sorted 
// order 
#include <bits/stdc++.h> 
using namespace std; 
  
char *removeDuplicate(char str[], int n) 
{ 
    // create a set using string characters 
    // excluding '\0' 
    set<char>s (str, str+n-1); 
  
    // print content of the set 
    int i = 0; 
    for (auto x : s) 
       str[i++] = x; 
    str[i] = '\0'; 
  
    return str; 
} 
  
// Driver code 
int main() 
{ 
   char str[]= "geeksforgeeks"; 
   int n = sizeof(str) / sizeof(str[0]); 
   cout << removeDuplicate(str, n); 
   return 0; 
} 

Output:

  efgkors

Time Complexity: O(n Log n)
Auxiliary Space: O(n)

Thanks to Anivesh Tiwari for suggesting this approach.

Does not keeps order of elements same as 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 same, then output should be geksfor, but above function returns efgkos. We can modify this method by storing the original order. METHOD 2 keeps the order same.

Implementation:

C++

// C++ prigram to remove duplicates, the order of 
// characters is not maintained in this program 
#include<bits/stdc++.h> 
using namespace std; 
  
/* Function to remove duplicates in a sorted array */
char *removeDupsSorted(char *str) 
{ 
    int res_ind = 1, ip_ind = 1; 
  
    /* In place removal of duplicate characters*/
    while (*(str + ip_ind)) 
    { 
        if (*(str + ip_ind) != *(str + ip_ind - 1)) 
        { 
            *(str + res_ind) = *(str + ip_ind); 
            res_ind++; 
        } 
        ip_ind++; 
    } 
  
    /* After above step string is efgkorskkorss. 
       Removing extra kkorss after string*/
    *(str + res_ind) = '\0'; 
  
    return str; 
} 
  
/* Function removes duplicate characters from the string 
   This function work in-place and fills null characters 
   in the extra space left */
char *removeDups(char *str) 
{ 
   int n = strlen(str); 
  
   // Sort the character array 
   sort(str, str+n); 
  
   // Remove duplicates from sorted 
   return removeDupsSorted(str); 
} 
  
/* Driver program to test removeDups */
int main() 
{ 
  char str[] = "geeksforgeeks"; 
  cout << removeDups(str); 
  return 0; 
} 

C

// C++ program to remove duplicates, the order of 
// characters is not maintained in this program 
# include <stdio.h> 
# include <stdlib.h> 
# include <string.h> 
/* Function to remove duplicates in a sorted array */
char *removeDupsSorted(char *str); 
  
/* Utitlity function to sort array A[] */
void quickSort(char A[], int si, int ei); 
  
/* Function removes duplicate characters from the string 
   This function work in-place and fills null characters 
   in the extra space left */
char *removeDups(char *str) 
{ 
  int len = strlen(str); 
  quickSort(str, 0, len-1); 
  return removeDupsSorted(str); 
}      
  
/* Function to remove duplicates in a sorted array */
char *removeDupsSorted(char *str) 
{ 
  int res_ind = 1, ip_ind = 1; 
  
  /* In place removal of duplicate characters*/
  while (*(str + ip_ind)) 
  { 
    if (*(str + ip_ind) != *(str + ip_ind - 1)) 
    { 
      *(str + res_ind) = *(str + ip_ind); 
      res_ind++; 
    } 
    ip_ind++; 
  }       
  
  /* After above step string is efgkorskkorss. 
     Removing extra kkorss after string*/
  *(str + res_ind) = '\0'; 
  
  return str; 
} 
  
/* Driver program to test removeDups */
int main() 
{ 
  char str[] = "geeksforgeeks"; 
  printf("%s", removeDups(str)); 
  getchar(); 
  return 0; 
} 
  
/* FOLLOWING FUNCTIONS ARE ONLY FOR SORTING 
    PURPOSE */
void exchange(char *a, char *b) 
{ 
  char temp; 
  temp = *a; 
  *a   = *b; 
  *b   = temp; 
} 
  
int partition(char A[], int si, int ei) 
{ 
  char x = A[ei]; 
  int i = (si - 1); 
  int j; 
  
  for (j = si; j <= ei - 1; j++) 
  { 
    if (A[j] <= x) 
    { 
      i++; 
      exchange(&A[i], &A[j]); 
    } 
  } 
  exchange (&A[i + 1], &A[ei]); 
  return (i + 1); 
} 
  
/* Implementation of Quick Sort 
A[] --> Array to be sorted 
si  --> Starting index 
ei  --> Ending index 
*/
void quickSort(char A[], int si, int ei) 
{ 
  int pi;    /* Partitioning index */
  if (si < ei) 
  { 
    pi = partition(A, si, ei); 
    quickSort(A, si, pi - 1); 
    quickSort(A, pi + 1, ei); 
  } 
} 

Java

// Java program to remove duplicates, the order of 
// characters is not maintained in this program 
  
import java.util.Arrays; 
  
public 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(); 
       Arrays.sort(temp); 
       str = new String(temp); 
         
       // Remove duplicates from sorted 
       return removeDupsSorted(str); 
    } 
       
    // Driver Method 
    public static void main(String[] args) 
    { 
        String str = "geeksforgeeks"; 
        System.out.println(removeDups(str)); 
    } 
} 

Python

# Python program to remove duplicates, the order of 
# characters is not maintained in this program 
  
# Utility function to convert string to list 
def toMutable(string): 
    temp = [] 
    for x in string: 
        temp.append(x) 
    return temp 
  
# Utility function to convert string to list 
def toString(List): 
    return ''.join(List) 
  
# Function to remove duplicates in a sorted array 
def removeDupsSorted(List): 
    res_ind = 1
    ip_ind = 1
  
    # In place removal of duplicate characters 
    while ip_ind != len(List): 
        if List[ip_ind] != List[ip_ind-1]: 
            List[res_ind] = List[ip_ind] 
            res_ind += 1
        ip_ind+=1
  
    # After above step string is efgkorskkorss. 
    # Removing extra kkorss after string 
    string = toString(List[0:res_ind]) 
  
    return string 
  
# Function removes duplicate characters from the string 
# This function work in-place and fills null characters 
# in the extra space left 
def removeDups(string): 
    # Convert string to list 
    List = toMutable(string) 
  
    # Sort the character list 
    List.sort() 
  
    # Remove duplicates from sorted 
    return removeDupsSorted(List) 
  
# Driver program to test the above functions 
string = "geeksforgeeks"
print removeDups(string) 
  
# This code is contributed by Bhavya Jain 

C#

// C# program to remove duplicates, the order of 
// characters is not maintained in this program 
using 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.

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 sringng" */
3: Remove extra characters at the end of the resultant string.
  /*  String obtained after this step is "te sring" */

Implementation:

C++

#include <bits/stdc++.h> 
using namespace std;  
# define NO_OF_CHARS 256  
# define bool int  
  
/* Function removes duplicate characters from the string  
This function work in-place and fills null characters  
in the extra space left */
char *removeDups(char str[])  
{  
    bool bin_hash[NO_OF_CHARS] = {0};  
    int ip_ind = 0, res_ind = 0;  
    char temp;  
      
    /* In place removal of duplicate characters*/
    while (*(str + ip_ind))  
    {  
        temp = *(str + ip_ind);  
        if (bin_hash[temp] == 0)  
        {  
            bin_hash[temp] = 1;  
            *(str + res_ind) = *(str + ip_ind);  
            res_ind++;  
        }  
        ip_ind++;  
    }  
      
    /* After above step string is stringiittg.  
        Removing extra iittg after string*/
    *(str+res_ind) = '\0';  
      
    return str;  
}  
  
/* Driver code */
int main()  
{  
    char str[] = "geeksforgeeks";  
    cout << removeDups(str);  
    return 0;  
}  
  
// This code is contributed by rathbhupendra 

C

# include <stdio.h> 
# include <stdlib.h> 
# define NO_OF_CHARS 256 
# define bool int 
  
/* Function removes duplicate characters from the string 
   This function work in-place and fills null characters 
   in the extra space left */
char *removeDups(char *str) 
{ 
  bool bin_hash[NO_OF_CHARS] = {0}; 
  int ip_ind = 0, res_ind = 0; 
  char temp;     
  
  /* In place removal of duplicate characters*/
  while (*(str + ip_ind)) 
  { 
    temp = *(str + ip_ind); 
    if (bin_hash[temp] == 0) 
    { 
        bin_hash[temp] = 1; 
        *(str + res_ind) = *(str + ip_ind); 
        res_ind++; 
    } 
    ip_ind++; 
  }       
  
  /* After above step string is stringiittg. 
     Removing extra iittg after string*/
  *(str+res_ind) = '\0';    
  
  return str; 
} 
  
/* Driver program to test removeDups */
int main() 
{ 
    char str[] = "geeksforgeeks"; 
    printf("%s", removeDups(str)); 
    getchar(); 
    return 0; 
} 

Java

// Java prigram to remove duplicates 
import java.util.*; 
  
class RemoveDuplicates 
{ 
    /* Function removes duplicate characters from the string 
    This function work in-place */
    void removeDuplicates(String str) 
    { 
        LinkedHashSet<Character> lhs = new LinkedHashSet<>(); 
        for(int i=0;i<str.length();i++) 
            lhs.add(str.charAt(i)); 
          
        // print string after deleting duplicate elements 
        for(Character ch : lhs) 
            System.out.print(ch); 
    } 
      
    /* Driver program to test removeDuplicates */
    public static void main(String args[]) 
    { 
        String str = "geeksforgeeks"; 
        RemoveDuplicates r = new RemoveDuplicates(); 
        r.removeDuplicates(str); 
    } 
} 
  
// This code has been contributed by Amit Khandelwal (Amit Khandelwal 1) 

Python

# Python program to remvoe duplicate characters from an 
# input string 
NO_OF_CHARS = 256
  
# Since strings in Python are immutable and cannot be changed 
# This utility function will convert the string to list 
def toMutable(string): 
    List = [] 
    for i in string: 
        List.append(i) 
    return List
  
# Utility function that changes list to string 
def toString(List): 
    return ''.join(List) 
  
# Function removes duplicate characters from the string 
# This function work in-place and fills null characters 
# in the extra space left 
def removeDups(string): 
    bin_hash = [0] * NO_OF_CHARS 
    ip_ind = 0
    res_ind = 0
    temp = '' 
    mutableString = toMutable(string) 
  
    # In place removal of duplicate characters 
    while ip_ind != len(mutableString): 
        temp = mutableString[ip_ind] 
        if bin_hash[ord(temp)] == 0: 
            bin_hash[ord(temp)] = 1
            mutableString[res_ind] = mutableString[ip_ind] 
            res_ind+=1
        ip_ind+=1
  
     # After above step string is stringiittg. 
     # Removing extra iittg after string 
    return toString(mutableString[0:res_ind]) 
  
# Driver program to test the above functions 
string = "geeksforgeeks"
print(removeDups(string)) 
  
# A shorter version for this program is as follows 
# import collections 
# print ''.join(collections.OrderedDict.fromkeys(string)) 
  
# This code is contributed by Bhavya Jain 

C#

// C# program to remove duplicates 
using 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 1 doesn’t maintain order of characters as original string, but method 2 does.
It is assumed that number of possible characters in input string are 256. NO_OF_CHARS should be changed accordingly.
calloc is used instead of malloc for memory allocations of counting array (count) to initialize allocated memory to ‘\0’. malloc() followed by memset() could also be used.
Above algorithm also works for an integer array inputs if range of the integers in array is given. Example problem is to find maximum occurring number in an input array given that the input array contain integers only between 1000 to 1100

Method 5 (Using IndexOf() method) :
Prerequisite : Java IndexOf() method

Java

// Java program too create a unique string 
import java.util.*; 
  
class IndexOf { 
      
    // Function to make the string unique 
    public static String unique(String s) 
    { 
        String str = new String(); 
        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.charAt(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"; 
          
        System.out.println(unique(s)); 
    } 
} 

C#

// C# program too create a unique string 
using 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++ program too 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. */
  
#include <bits/stdc++.h>  
using namespace std;  
char* removeDuplicates(char *s,int n){ 
  unordered_map<char,int> exists; 
  int index = 0;  
  for(int i=0;i<n;i++){ 
    if(exists[s[i]]==0) 
    { 
      s[index++] = s[i]; 
      exists[s[i]]++; 
    } 
  } 
  return s; 
} 
  
//driver code 
int main(){ 
  char s[] = "geeksforgeeks"; 
  int n = sizeof(s)/sizeof(s[0]); 
  cout<<removeDuplicates(s,n)<<endl; 
  return 0; 
} 

Output:

geksfor

Time Complexity : O(n)
Auxiliary Space : O(n)

Thanks Allen James Vinoy for suggesting this approach.

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