Remove duplicates from a string in O(1) extra space

Given a string str of lowercase characters, the task is to remove duplicates and return a resultant string without modifying the order of characters in the original string.

Examples:

Input: str = "geeksforgeeks"
Output: geksfor

Input: str = "characters"
Output: chartes

Approach: The idea is to use bits of a counter variable to mark the presence of a character in the string. To mark the presence of ‘a’ set 0th bit as 1, for ‘b’ set 1st bit as 1 and so on. If the corresponding bit of character present in the original string is set to 0, it means it is the first occurrence of that character, hence set its corresponding bit as 1 and keep on including the current character in the resultant string.

Consider the string str = “geeksforgeeks”

  • character: ‘g’
    x = 6(ascii of g – 97)
    6th bit in counter is unset resulting first occurrence of character ‘g’.
    str[0] = ‘g’
    counter = 00000000000000000000000001000000 // mark 6th bit as visited
    length = 1
  • character: ‘e’
    x = 4(ascii of e – 97)
    4th bit in counter is unset resulting in first occurrence of character ‘e’.
    str[1] = ‘e’
    counter = 00000000000000000000000001010000 //mark 4th bit as visited
    length = 2
  • character: ‘e’
    x = 4(ascii of e – 97)
    4th bit in counter is set resulting in duplicate character.
    Ignore this character. Move for next character.
    counter = 00000000000000000000000001010000 //same as previous
    length = 2
  • character: ‘k’
    x = 10(ascii of k – 97)
    10th bit in counter is unset resulting in first occurrence of character ‘k’.
    str[2] = ‘k’
    counter = 00000000000000000000010001010000 //mark 10th bit as visited
    length = 3

Similarly, do the same for all characters.

Resultant string : geksfor(string of length 7 starting from index 0)

Algorithm:

  1. Initialize a counter variable (keeps track of the characters visited in string), it is a 32 bit Integer represented as(00000000000000000000000000000000) initially.
  2. Consider ‘a’ as 0th bit of counter, ‘b’ as 1st bit of counter, ‘c’ as 2nd bit of counter and so on.
  3. Traverse through each character of input string.
  4. Get the character’s value, where character’s value(x) = Ascii of character – 97. This will make sure for value of ‘a’ as 0, value of ‘b’ as 1 and so on.
  5. Check xth bit of counter.
  6. If Xth bit of counter is 0 which means the current character has appeared for the first time, keep the current character at the index “length” of string .
  7. Mark the current character visited by setting xth bit of counter.
  8. Increment length.
  9. Return Substring of size “length” from index 0.

Below is the implementation of above approach:

C++

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// C++ implementation of above approach
#include <bits/stdc++.h>
#include <string>
using namespace std;
  
// Function to remove duplicates
string removeDuplicatesFromString(string str)
{
  
    // keeps track of visited characters
    int counter = 0;
  
    int i = 0;
    int size = str.size();
  
    // gets character value
    int x;
  
    // keeps track of length of resultant string
    int length = 0;
  
    while (i < size) {
        x = str[i] - 97;
  
        // check if Xth bit of counter is unset
        if ((counter & (1 << x)) == 0) {
  
            str[length] = 'a' + x;
  
            // mark current character as visited
            counter = counter | (1 << x);
  
            length++;
        }
        i++;
    }
  
    return str.substr(0, length);
}
  
// Driver code
int main()
{
    string str = "geeksforgeeks";
    cout << removeDuplicatesFromString(str);
    return 0;
}

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Java

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// Java implementation of above approach
import java.util.Arrays;
  
class GFG {
  
    // Function to remove duplicates
    static char[] removeDuplicatesFromString(String string)
    {
  
        // keeps track of visited characters
        int counter = 0;
        char[] str = string.toCharArray();
        int i = 0;
        int size = str.length;
  
        // gets character value
        int x;
  
        // keeps track of length of resultant String
        int length = 0;
  
        while (i < size) {
            x = str[i] - 97;
  
            // check if Xth bit of counter is unset
            if ((counter & (1 << x)) == 0) {
  
                str[length] = (char)('a' + x);
  
                // mark current character as visited
                counter = counter | (1 << x);
  
                length++;
            }
            i++;
        }
  
        return Arrays.copyOfRange(str, 0, length);
    }
  
    // Driver code
    public static void main(String[] args)
    {
        String str = "geeksforgeeks";
        System.out.println(removeDuplicatesFromString(str));
    }
}
  
// This code is contributed by Mithun Kumar

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Python3

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# Python3 implementation of above approach
  
# Function to remove duplicates
def removeDuplicatesFromString(str2):
  
    # keeps track of visited characters
    counter = 0;
  
    i = 0;
    size = len(str2);
    str1 = list(str2);
  
    # gets character value
    x = 0;
  
    # keeps track of length of resultant string
    length = 0;
  
    while (i < size):
        x = ord(str1[i]) - 97;
  
        # check if Xth bit of counter is unset
        if ((counter & (1 << x)) == 0):
            str1[length] = chr(97 + x);
  
            # mark current character as visited
            counter = counter | (1 << x);
  
            length += 1;
        i += 1;
          
    str2=''.join(str1);
    return str2[0:length];
  
# Driver code
str1 = "geeksforgeeks";
print(removeDuplicatesFromString(str1));
  
# This code is contributed by mits

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C#

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// C# implementation of above approach
using System;
  
class GFG {
  
    // Function to remove duplicates
    static string removeDuplicatesFromString(string string1)
    {
  
        // keeps track of visited characters
        int counter = 0;
        char[] str = string1.ToCharArray();
        int i = 0;
        int size = str.Length;
  
        // gets character value
        int x;
  
        // keeps track of length of resultant String
        int length = 0;
  
        while (i < size) {
            x = str[i] - 97;
  
            // check if Xth bit of counter is unset
            if ((counter & (1 << x)) == 0) {
  
                str[length] = (char)('a' + x);
  
                // mark current character as visited
                counter = counter | (1 << x);
  
                length++;
            }
            i++;
        }
  
        return (new string(str)).Substring(0, length);
    }
  
    // Driver code
    static void Main()
    {
        string str = "geeksforgeeks";
        Console.WriteLine(removeDuplicatesFromString(str));
    }
}
  
// This code is contributed by mits

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PHP

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<?php
// PHP implementation of above approach
  
// Function to remove duplicates
function removeDuplicatesFromString($str)
{
  
    // keeps track of visited characters
    $counter = 0;
  
    $i = 0;
    $size = strlen($str);
  
    // gets character value
    $x = 0;
  
    // keeps track of length of resultant string
    $length = 0;
  
    while ($i < $size
    {
        $x = ord($str[$i]) - 97;
  
        // check if Xth bit of counter is unset
        if (($counter & (1 << $x)) == 0) 
        {
            $str[$length] = chr(97 + $x);
  
            // mark current character as visited
            $counter = $counter | (1 << $x);
  
            $length++;
        }
        $i++;
    }
  
    return substr($str, 0, $length);
}
  
// Driver code
$str = "geeksforgeeks";
echo removeDuplicatesFromString($str);
  
// This code is contributed by mits
?>

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Output:

geksfor

Time Complexity: O(n)
Space Complexity: O(1)

Another Approach: This approach keeps track of visited characters from given input string through an integer array of size 256 (All possible characters).

The idea is as follows:

  1. Create an integer array of size 256 in order to keep track of all possible characters.
  2. Iterate over the input string, and for each character :
  3. Lookup into the array with the ASCII value of character as index:
    • If value at index is 0, then copy the character into original input array and increase the endIndex also update the value at index as -1.
    • Else skip the character.

Below is the implementation of the above approach:

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//Java implementation of above approach 
import java.util.Arrays; 
  
class GFG { 
  
    // Method to remove duplicates 
    static char[] removeDuplicatesFromString(String string) 
    
        //table to keep track of visited characters
        int[] table = new int[256];
        char[] chars = string.toCharArray();
  
        //to keep track of end index of resultant string
        int endIndex = 0;
      
        for(int i = 0; i < chars.length; i++)
        {
            if(table[chars[i]] == 0)
            {
                table[chars[i]] = -1;
                chars[endIndex++] = chars[i]; 
            }
        }
      
        return Arrays.copyOfRange(chars, 0, endIndex);
    
  
    // Driver code 
    public static void main(String[] args) 
    
        String str = "geeksforgeeks"
        System.out.println(removeDuplicatesFromString(str)); 
    
//This code is contributed by Sonu Singh 

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Output:

geksfor

Time Complexity: O(n)
Space Complexity: O(1)

This approach is contributed by Sonu Singh.



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