找出数组中的幸运数

/*
 * Solution approach using Java Streams:
 * 1. Create a frequency map using the stream's collect method and a counting collector.
 * 2. Use the entrySet stream to filter entries where the key equals the value (lucky number condition).
 * 3. Use the max operation to find the largest lucky number, or return -1 if none exist.
 */
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class LuckyNumberStream {
    public int findLucky(int[] arr) {
        Map<Integer, Long> frequencyMap = IntStream.of(arr)
                .boxed()
                .collect(Collectors.groupingBy(num -> num, Collectors.counting()));
        return frequencyMap.entrySet().stream()
                .filter(entry -> entry.getKey().equals(entry.getValue().intValue()))
                .map(Map.Entry::getKey)
                .max(Integer::compare)
                .orElse(-1);
    }
}