Efficiently check if a string has all unique characters without using any additional data structure

Implement an space efficient algorithm to determine if a string (of characters from ‘a’ to ‘z’) has all unique characters or not. Use additional data structures like count array, hash, etc is not allowed.
Expected Time Complexity : O(n) 

Examples :

Input  : str = "aaabbccdaa"
Output : No

Input  : str = "abcd"
Output : Yes

Brute Force Approach:

The brute force approach to solve this problem is to compare each character of the string with all the other characters in the string to check if it is unique or not. We can define a function that takes the input string and returns true if all the characters in the string are unique, and false otherwise.

1. Traverse the string character by character.
2. For each character, compare it with all the other characters in the string.
3. If any other character is found to be equal to the current character, return false as the string has duplicate characters.
4. If the end of the string is reached without finding any duplicate characters, return true as the string has all unique characters.

Below is the implementation of the above approach:

C++

#include<bits/stdc++.h>
using namespace std;

bool areChractersUnique(string str)
{
    int n = str.length();
    for(int i = 0; i < n; i++)
        for(int j = i + 1; j < n; j++)
            if(str[i] == str[j])
                return false;
    return true;
}

int main()
{
    string s = "aaabbccdaa";
    if (areChractersUnique(s))
        cout << "Yes";
    else
        cout << "No";
    return 0;
}

Java

import java.util.*;

public class Main {
  public static boolean areCharactersUnique(String str) {
    int n = str.length();
    for (int i = 0; i < n; i++)
      for (int j = i + 1; j < n; j++)
        if (str.charAt(i) == str.charAt(j))
          return false;
    return true;
  }

  public static void main(String[] args) {
    String s = "aaabbccdaa";
    if (areCharactersUnique(s))
      System.out.println("Yes");
    else
      System.out.println("No");
  }
}

Python3

# This function checks if all characters in a string are unique.
def are_characters_unique(string):

    n = len(string)
    for i in range(n):
        for j in range(i + 1, n):
            if string[i] == string[j]:
                return False
    return True


if __name__ == '__main__':
    s = "aaabbccdaa"
    if are_characters_unique(s):
        print("Yes")
    else:
        print("No")

C#

using System;

public class GFG
{
    public static bool areChractersUnique(string str)
    {
        int n = str.Length;
        for (int i = 0; i < n; i++)
            for (int j = i + 1; j < n; j++)
                if (str[i] == str[j])
                    return false;
        return true;
    }

    public static void Main()
    {
        string s = "aaabbccdaa";
        if (areChractersUnique(s))
            Console.WriteLine("Yes");
        else
            Console.WriteLine("No");
    }
}

Javascript

// javascript code addition

// This function checks if all characters in a string are unique.
function areChractersUnique(str)
{

    // Find the length of the string
    let n = str.length;

    // outer loop
    for(let i = 0; i < n; i++)
        // innerloop
        for(let j = i + 1; j < n; j++)
            if(str[i] == str[j])
                return false;
    return true;
}

let s = "aaabbccdaa";
if (areChractersUnique(s))
    // Print "Yes" if all characters are unique.
    console.log("Yes");
else
    console.log("No");

// The code is contributed by Arushi Goel.

Output

No

Time Complexity: O(N^2)

Auxiliary Space: O(1)

The idea is to use an integer variable and use bits in its binary representation to store whether a character is present or not. Typically an integer has at-least 32 bits and we need to store presence/absence of only 26 characters.

Below is the implementation of the idea.

C++

// A space efficient C++ program to check if
// all characters of string are unique.
#include<bits/stdc++.h>
using namespace std;

// Returns true if all characters of str are
// unique.
// Assumptions : (1) str contains only characters
//                   from 'a' to 'z'
//               (2) integers are stored using 32
//                   bits
bool areChractersUnique(string str)
{
    // An integer to store presence/absence
    // of 26 characters using its 32 bits.
    int checker = 0;

    for (int i = 0; i < str.length(); ++i)
    {
        int val = (str[i]-'a');

        // If bit corresponding to current
        // character is already set
        if ((checker & (1 << val)) > 0)
            return false;

        // set bit in checker
        checker |=  (1 << val);
    }

    return true;
}

// Driver code
int main()
{
    string s = "aaabbccdaa";
    if (areChractersUnique(s))
        cout << "Yes";
    else
        cout << "No";
    return 0;
}

Java

// A space efficient Java program to check if
// all characters of string are unique.
class GFG {

    // Returns true if all characters of str are
    // unique.
    // Assumptions : (1) str contains only characters
    //                 from 'a' to 'z'
    //             (2) integers are stored using 32
    //                 bits
    static boolean areChractersUnique(String str)
    {

        // An integer to store presence/absence
        // of 26 characters using its 32 bits.
        int checker = 0;

        for (int i = 0; i < str.length(); ++i)
        {
            int val = (str.charAt(i)-'a');

            // If bit corresponding to current
            // character is already set
            if ((checker & (1 << val)) > 0)
                return false;

            // set bit in checker
            checker |= (1 << val);
        }

        return true;
    }

    //driver code
    public static void main (String[] args)
    {
        String s = "aaabbccdaa";

        if (areChractersUnique(s))
            System.out.print("Yes");
        else
            System.out.print("No");
    }
}

// This code is contributed by Anant Agarwal.

Python3

# A space efficient Python3 program to check if
# all characters of string are unique

# Returns true if all characters of str are
# unique.
# Assumptions : (1) str contains only characters
#                    from 'a' to 'z'
#                (2) integers are stored using 32
#                    bits

def areCharactersUnique(s):

    # An integer to store presence/absence
    # of 26 characters using its 32 bits
    checker = 0

    for i in range(len(s)):

        val = ord(s[i]) - ord('a')

        # If bit corresponding to current
        # character is already set
        if (checker & (1 << val)) > 0:
            return False

        # set bit in checker
        checker |= (1 << val)

    return True

# Driver code
s = "aaabbccdaa"
if areCharactersUnique(s):
    print("Yes")
else:
    print("No")

# This code is contributed
# by Mohit Kumar

C#

// A space efficient program
// to check if all characters
// of string are unique.
using System;

class GFG {

    // Returns true if all characters
    // of str are unique. Assumptions:
    // (1)str contains only characters
    // from 'a' to 'z'.(2)integers are
    // stored using 32 bits
    static bool areChractersUnique(string str)
    {
        // An integer to store presence
        // or absence of 26 characters
        // using its 32 bits.
        int checker = 0;

        for (int i = 0; i < str.Length; ++i) {
            int val = (str[i] - 'a');

            // If bit corresponding to current
            // character is already set
            if ((checker & (1 << val)) > 0)
                return false;

            // set bit in checker
            checker |= (1 << val);
        }

        return true;
    }

    // Driver code
    public static void Main()
    {
        string s = "aaabbccdaa";

        if (areChractersUnique(s))
            Console.Write("Yes");
        else
            Console.Write("No");
    }
}

// This code is contributed by Anant Agarwal.

PHP

<?php
// A space efficient PHP program
// to check if all characters of
// string are unique.

// Returns true if all characters
// of str are unique.
// Assumptions : (1) str contains
//                     only characters
//                   from 'a' to 'z'
//         (2) integers are stored
//             using 32 bits
function areChractersUnique($str)
{
    // An integer to store presence/absence
    // of 26 characters using its 32 bits.
    $checker = 0;

    for ($i = 0; $i < $len = strlen($str); ++$i)
    {
        $val = ($str[$i] - 'a');

        // If bit corresponding to current
        // character is already set
        if (($checker & (1 << $val)) > 0)
            return false;

        // set bit in checker
        $checker |= (1 << $val);
    }

    return true;
}

// Driver code
$s = "aaabbccdaa";
if (areChractersUnique($s))
    echo "Yes";
else
    echo "No";

// This code is contributed by aj_36
?>

Javascript

<script>

// Javascript program for the above approach

    // Returns true if all characters of str are
    // unique.
    // Assumptions : (1) str contains only characters
    //                   from 'a' to 'z'
    //               (2) integers are stored using 32
    //                 bits
    function areChractersUnique(str)
    {

        // An integer to store presence/absence
        // of 26 characters using its 32 bits.
        let checker = 0;

        for (let i = 0; i < str.length; ++i)
        {
            let val = (str[i]-'a');

            // If bit corresponding to current
            // character is already set
            if ((checker & (1 << val)) > 0)
                return false;

            // set bit in checker
            checker |= (1 << val);
        }

        return true;
    }

// Driver Code

    var s = "aaabbccdaa";

    if (areChractersUnique(s))
        document.write("Yes");
    else
        document.write("No");

</script>

Output

No

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

Another Implementation: Using STL

C++

#include <bits/stdc++.h>
using namespace std;
bool unique(string s) {
    sort(s.begin(),s.end());
    for(int i=0;i<s.size()-1;i++)
    {
        if(s[i]==s[i+1])
        {
            return false;
            break;
        }
    }
    return true;
}
int main() {
  if(unique("abcdd")==true)
  {

    cout <<"String is Unique"<<endl;
  }
  else
  {
    cout <<"String is not Unique"<<endl;
  }

    return 0;
}

Java

import java.util.Arrays;

class GFG {

    static boolean unique(String s)
    {
        Arrays.sort(s.toCharArray());
        for (int i = 0; i < s.length()-1; i++) {
            if (s.charAt(i) == s.charAt(i + 1)) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args)
    {
        if (unique("abcdd") == true) {
            System.out.println("String is Unique");
        }
        else {
            System.out.println("String is not Unique");
        }
    }
}

// This code is contributed by rajsanghavi9.

Python3

def unique(s):
    s = list(s)
    s.sort()
    for i in range (len(s) - 1):

        if(s[i] == s[i + 1]):

            return False
            break

    return True

if(unique("abcdd") == True):
  print("String is Unique")

else:
  print("String is not Unique")

# This code is contributed by shivanisinghss2110

C#

using System;

public class GFG {

    static bool unique(String s) {
        Array.Sort(s.ToCharArray());
        for (int i = 0; i < s.Length-1; i++) {
            if (s[i] == s[i + 1]) {
                return false;
            }
        }
        return true;
    }

  // Driver code
    public static void Main(String[] args) {
        if (unique("abcdd") == true) {
            Console.WriteLine("String is Unique");
        } else {
            Console.WriteLine("String is not Unique");
        }
    }
}

// This code is contributed by umadevi9616

Javascript

<script>
    function unique(s) {

        for (var i = 0; i < s.length-1; i++) {
            if (s.charAt(i) == s.charAt(i + 1)) {
                return false;
            }
        }
        return true;
    }


        if (unique("abcdd") == true) {
            document.write("String is Unique");
        } else {
            document.write("String is not Unique");
        }

// This code is contributed by umadevi9616.
</script>

Output

String is not Unique

Time Complexity : O(nlogn), where n is the length of the given string.
Auxiliary Space : O(1), no extra space is required so it is a constant.

This article is contributed by Mr. Somesh Awasthi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.