最大唯一数

/*
 * Leetcode 1133: Largest Unique Number
 * 
 * Given an array of integers, return the largest integer that only occurs once. If no integer occurs once, return -1.
 * 
 * Approach using Java Stream:
 * 1. Use a HashMap to count the occurrences of each number.
 * 2. Use streams to filter out numbers that appear more than once and find the maximum of the remaining numbers.
 * 3. If no such number exists, return -1.
 */

import java.util.HashMap;
import java.util.Map;
import java.util.OptionalInt;
import java.util.stream.IntStream;

public class LargestUniqueNumberStream {
    public int largestUniqueNumber(int[] nums) {
        Map<Integer, Integer> countMap = new HashMap<>();
        for (int num : nums) {
            countMap.put(num, countMap.getOrDefault(num, 0) + 1);
        }
        OptionalInt max = countMap.entrySet().stream()
            .filter(entry -> entry.getValue() == 1)
            .mapToInt(Map.Entry::getKey)
            .max();
        return max.orElse(-1);
    }
}