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

Difficulty Level : Hard
Last Updated : 03 Jun, 2021

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

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 efficeint c++ 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();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;
}

Output

String is not Unique

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