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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

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.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

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