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:

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.

No

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

Another Implementation: Using STL

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.