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

O(1) Algorithm

Your solution is O(n^2) because of the two nested loops.

The fastest algorithm to do this is O(1) (constant time):

public static int detectDuplicate(String source) {
    boolean[] foundChars = new boolean[Character.MAX_VALUE+1];
    for(int i = 0; i < source.length(); i++) {
        if(i >= Character.MAX_VALUE) return Character.MAX_VALUE;
        char currentChar = source.charAt(i);
        if(foundChars[currentChar]) return i;
        foundChars[currentChar] = true;
    }
    return -1;
}

However, this is only fast in terms of big oh.