Java - What is the best way to find first duplicate character in a string

Question

I have written below code for detecting first duplicate character in a string.

public static int detectDuplicate(String source) {
    boolean found = false;         


        

Answer 1

Okay, I found below logic to reduce O(N^2) to O(N).

public static int detectDuplicate(String source) {
    int index = -1;
    boolean found = false;
    final long start = System.currentTimeMillis();

    for(int i = 1; i <= source.length() && !found; i++) {
        if(source.charAt(i) == source.charAt(i-1)) {
            index = (i - 1);
            found = true;
        }
    }

    System.out.println("Time taken --> " + (System.currentTimeMillis() - start) + " ms. for string of length --> " + source.length());
    return index;
}

This also shows performance improvement over my previous algorithm which has 2 nested loops. This takes 130ms. to detect first duplicate character from 63million characters where the duplicate character is present at the end.

I am not confident if this is the best solution. If anyone finds a better one, please please share it.

Thanks,

NN