数组中第一个不重复的字符（Java）

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String[] arr = new String[]{"a","b","a","c","c"};
        System.out.println(f1(arr));
        System.out.println(f2(arr));
    }
    public static String f1(String[] arr){
        int index=0;
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr.length; j++) {
                if(arr[i]==arr[j]){
                    index++;
                }
            }
            if(index==1){
                return arr[i];
            }else{
                index=0;
            }
        }
        return "not at all";
    }


    public static String f2(String[] arr ){
        HashMap map = new HashMap<>();
        for (int i = 0; i < arr.length; i++) {
            String s = arr[i];
            map.put(s,0);
        }
        for (int i = 0; i < arr.length; i++) {
            String s = arr[i];
            map.put(s,map.get(s)+1);
        }
        for (int i = 0; i < arr.length; i++) {
            if(map.get(arr[i])==1){
                return arr[i];
            }
        }
        return "都是重复的";
    }
}