Remove duplicates from a string

Given a string s which may contain lowercase and uppercase characters. The task is to remove all duplicate characters from the string and find the resultant string.

Note: The order of remaining characters in the output should be the same as in the original string.

Example:

Input: s = geeksforgeeks
Output: geksfor
Explanation: After removing duplicate characters such as e, k, g, s, we have string as "geksfor".

Input: s = HappyNewYear
Output: HapyNewYr
Explanation: After removing duplicate characters such as p, e, a, we have string as "HapyNewYr".

Naive Approach - O(n^2) Time

Iterate through the string and for each character check if that particular character has occurred before it in the string. If not, add the character to the result, otherwise the character is not added to result.

Below is the implementation of above approach:

geksfor

Time Complexity: O(n * n) 
Auxiliary Space: O(1), Keeps the order of elements the same as the input. 

Using Hash Set - O(n) Time

Iterating through the given string and use a map to efficiently track of encountered characters. If a character is encountered for the first time, it's added to the result string, Otherwise, it's skipped. This ensures the output string contains only unique characters in the same order as the input string.

Below is the implementation of above approach:

geksfor

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

Using Frequency Array - O(n)

Iterating through the given string and use a character array initilize with 0 frequency to efficiently track of encountered characters. If current character's frequency is 0, then it's added to the result string and increment frequency by 1, Otherwise, it's skipped. This ensures the output string contains only unique characters in the same order as the input string.

Below is the implementation of above approach:

geksfor

Time Complexity: O(n)
Auxiliary Space: O(1) considering that the alphabet size is constant.