Missing Ranges leetcode

Given a sorted integer array where the range of elements are [lower, upper] inclusive, 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"].

public class Solution {
    public List<String> findMissingRanges(int[] nums, int lower, int upper) {
        List<String> res = new ArrayList<String>();
        if (nums.length == 0) {
            res.add(helper(lower, upper));
            return res;
        }
        if (nums[0] > lower) {
            res.add(helper(lower, nums[0] - 1));
        }
        for (int i = 1; i < nums.length; i++) {
            if (nums[i] - nums[i - 1] > 1) {
                res.add(helper(nums[i - 1] + 1, nums[i] - 1));
            }
        }
        if (nums[nums.length - 1] < upper) {
            res.add(helper(nums[nums.length - 1] + 1, upper));
        }
        return res;
    }
    public String helper(int lower, int upper) {
        if (lower == upper) {
            return lower + "";
        } else {
            return lower + "->" + upper;
        }
    }
}