Given a sorted integer array where the range of elements are in the inclusive range [lower, upper], return its missing ranges.

For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

思路：
查看每个数字和上一个数字中是否需要补区间，第一个数字需要和lower做比较，最后一个数字还要和upper做一次比较。
此题坑较多：比如low和up相同，数组为空的case；low和up是int类型的最小和最大值的情况，此时用num[i] - num[i-1]就会有溢出。

public static List<String> findMissingRanges(int[] nums, int lower, int upper) {
    List<String> res = new ArrayList<>();
    if (nums == null || nums.length == 0) {
        String range = "";
        if (lower == upper) {
            range = String.valueOf(lower);
        } else {
            range = String.valueOf(lower) + "->" + String.valueOf(upper);
        }
        //String range = String.valueOf(lower) + "->" + String.valueOf(upper);//bug
        res.add(range);
        return res;
    }

    for (int i = 0; i < nums.length; i++) {
        String range = "";
        //if (i == 0 && nums[i] > lower) {//bug//第一个和lower比
        if (i == 0) {//第一个和lower比
            if (nums[i] > lower) {
                if (nums[i] - 1 == lower) {
                    range = String.valueOf(lower);
                } else {
                    range = String.valueOf(lower) + "->" + String.valueOf(nums[i] - 1);
                }
            }
        } else {
            long diff = (long)nums[i] - (long)nums[i-1];
            System.out.println(diff);
            if (diff == 2) {//其余的和前一个比
                range = String.valueOf(nums[i] - 1);
            } else if (diff > 2) {
                range = String.valueOf(nums[i-1] + 1) + "->" + String.valueOf(nums[i] - 1);
            }
        }
        if (!range.equals("")) {
            res.add(range);
        }
        //res.add(range);//bug 会向res重复add range

        if (i == nums.length - 1 && nums[i] < upper) {//最后一个再和upper比一次
            if (upper - 1 == nums[i]) {
                range = String.valueOf(upper);
            } else {
                range = String.valueOf(nums[i] + 1) + "->" + String.valueOf(upper);
            }
            res.add(range);
        }
        //res.add(range); //bug 会向res重复add range
    }

    return res;
}