Q2

import java.util.Arrays;
import java.util.Random;

public class UniqueRandomNumberGenerator {
public static void main(String[] args) {
int count = 5000000; // 生成500万个随机数
int[] uniqueRandomNumbers = generateUniqueRandomNumbers(count);

    // 打印前10个随机数来验证
    for (int i = 0; i < 10; i++) {
        System.out.println(uniqueRandomNumbers[i]);
    }
}

public static int[] generateUniqueRandomNumbers(int count) {
    if (count > 5000000) {
        throw new IllegalArgumentException("Count cannot exceed the range of 0 to 5000000.");
    }

    int[] numbers = new int[count];
    for (int i = 0; i < count; i++) {
        numbers[i] = i; // 初始化数组为0, 1, ..., count-1
    }

    Random random = new Random();
    for (int i = count - 1; i > 0; i--) {
        int index = random.nextInt(i + 1); // 生成0到i之间的随机索引
        // 交换numbers[i]和numbers[index]
        int temp = numbers[i];
        numbers[i] = numbers[index];
        numbers[index] = temp;
    }

    return numbers;
}

}