import java.util.LinkedHashMap;
import java.util.Map;

import static java.util.function.Function.identity;
import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

public class NonRepeatingLetter {
    public static void main(String[] args) {
        System.out.println(findFirstNonRepeatingLetter(args[0]));
    }

    // "teeter"   = 'r'
    // "stress"   = 't'
    // "choochoo" = Exception
    // ""         = Exception
    public static char findFirstNonRepeatingLetter(final String string) {
                // transform string to a IntStream
        return string.chars()
                // map to Character to use object collectors
                .mapToObj(i -> (char) i)                                                // avoid unnecessary wrapping
                // collect the Characters into a LinkedHashMap (to maintain insertion order) grouping by their count
                .collect(groupingBy(identity(), LinkedHashMap::new, counting()))
                // transform Map<Character, Long> into a Stream<Entry<Character, Long>>
                .entrySet().stream()
                // filter by 1 occurrence
                .filter(e -> e.getValue() == 1)
                // map to the Character
                .map(Map.Entry::getKey)                                                 // use a method reference
                // find the first Character that satisfies the filter
                .findFirst()
                // if no Character is found, throw an exception
                .orElseThrow(() -> new RuntimeException("Only duplicates present"));    // handle all duplicates or empty
    }
}