Space Complexity: O(1), as we are not using any additional data structures that grow with the input size. We are only using a constant amount of space for the loop variables.

Optimal Approach - 1

Algorithm

Create a frequency array of size 256 to store the count of all characters (based on their ASCII values).
Traverse the string from left to right, and for each character, increment its count in the frequency array.
Once the entire string is traversed, start another loop that iterates over the frequency array.
If any value in the frequency array is greater than 1, return the index, and by typecasting the index to a character, obtain the first repeating character.

Code

123456789101112131415161718192021222324252627282930313233343536373839404142434445