Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

I have two strings i.e. 'unique' and 'repeated'. Every character appearing for the first time, gets added to 'unique'. If it is repeated for the second time, it gets removed from 'unique' and added to 'repeated'. This way, we will always have a string of unique characters in 'unique'. Complexity big O(n)

public void firstUniqueChar(String str){
    String unique= "";
    String repeated = "";
    str = str.toLowerCase();
    for(int i=0; i