找出数组中重复的值

public class RepeatBase {

    public static int[] num = {1,0,8,4,5,6,2,8,1,6};

    public static void main(String[] args) {
       // System.out.println(repeatCheckBySet(num));
       // System.out.println(repeatCheckByTemp(num));
       // System.out.println(repeatCheckByOrderSort(num));
        System.out.println(isPowerOfTwo(7));
    }

    /**
     * 时间复杂数为O(n)-线性
     * 利用set保存元素的特性，存在-不保存-返回false，不存在-保存-返回true
     *
     * @param num
     * @return
     */
    public static int repeatCheckBySet(int[] num) {
        //对原数据进行严格校验
        if (num == null || num.length == 0) {
            return -1;
        }
        Set<Integer> setCheck = new HashSet<>();
        for (int source : num) {
            if (!setCheck.add(source)) {
                return source;
            }
        }
        return -1;
    }

    /**
     * 借助temp，索引位置累计加一，然后判断是否有重复
     *
     * @param num
     * @return
     */
    public static String repeatCheckByTemp(int[] num) {
        if (num == null || num.length == 0) {
            return "-1";
        }
        StringBuilder stringBuilder = new StringBuilder();
        int[] temp = new int[num.length];
        for (int source : num) {
            temp[source]++;
            if (temp[source] > 1) {
                stringBuilder.append(source).append("_");
                continue;
            }
        }
        return StringUtils.isBlank(stringBuilder.toString()) ? "-1" : stringBuilder.deleteCharAt(stringBuilder.length() - 1).toString();
    }

    /**
     * 排序后，进行相邻之间的比较，相等及返回
     *
     * @param num
     * @return
     */
    public static int repeatCheckByOrderSort(int[] num) {
        if (num == null || num.length == 0) {
            return -1;
        }
        Arrays.sort(num);
        int length = num.length;
        for (int i = 0; i + 1 < length; i++) {
            if (num[i] == num[i + 1]) {
                return num[i];
            }
        }
        return -1;
    }

    /**
     * == 优先级高于 &
     * 求某个数是否是2的幂等值
     *
     * @param n
     * @return
     */
    public static boolean isPowerOfTwo(int n) {
        return (n & (n - 1)) == 0;
    }

}