Find the duplicate characters in a string in O(1) space

Given a stringstr, the task is to find all the duplicate characters present in a given string in lexicographical order without using any additional data structure.

Examples:

Input: str = "geeksforgeeks" 
Output: e g k s 
Explanation:
Frequency of character 'g' = 2 
Frequency of character 'e' = 4 
Frequency of character 'k' = 2 
Frequency of character 's' = 2 
Therefore, the required output is e g k s.

Input: str = "apple" 
Output: p 
Explanation:
Frequency of character 'p' = 2. 
Therefore, the required output is p.

Approach: Follow the steps below to solve the problem:

• Initialize a variable, say first, where i^th bit of first check if the character (i + 'a') present in the string at least once or not.
• Initialize a variable, say second, where i^th bit of second check if the character (i + 'a') present in the string at least twice or not.
• Iterate over the characters of the string. For every i^th character, check if str[i] has already occurred in the string or not. If found to be true, then set the (str[i] - 'a')^th bit of second.
• Otherwise, set (str[i] - 'a')^th bit of first.
• Finally, iterate over the range [0, 25] and check if i^th bit of both first and second is set or not. If found to be true, then print (i + 'a').

Below is the implementation of the above approach:

e g k s

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

Approach: Using Sorting

Steps:

• First, sort the string.
• Then compare adjacent characters to find duplicates.
• Last return the result.

e g k s

Time Complexity: O(nlog(n)), where n is the length of the input string .

Auxiliary Space: O(n), where n is the length of the input string.