LeetCode 163 Missing Ranges

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"].

這題的follow up個人感覺會非常復雜。。。如果需要考慮給出的[lower, upper]區間否是完全包含sorted array。。。

不過這里說了ranges是[lower, upper] inclusive，所以只考慮sorted array完全在該范圍內的情況，因此需要重點考慮的是lower和upper兩頭的case。

自己寫了一下代碼比較繁瑣。。。

代碼：

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