Efficiently check if a string has duplicates 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)

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 C++ implementation of the idea.

// 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;
}

Output:

No

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

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

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