0163. Missing Ranges

https://leetcode.com/problems/missing-ranges

Description

You are given an inclusive range [lower, upper] and a sorted unique integer array nums, where all elements are in the inclusive range.

A number x is considered missing if x is in the range [lower, upper] and x is not in nums.

Return the smallest sorted list of ranges that cover every missing number exactly. That is, no element of nums is in any of the ranges, and each missing number is in one of the ranges.

Each range [a,b] in the list should be output as:

• "a->b" if a != b

• "a" if a == b

Example 1:

**Input:** nums = [0,1,3,50,75], lower = 0, upper = 99
**Output:** ["2","4->49","51->74","76->99"]
**Explanation:** The ranges are:
[2,2] --> "2"
[4,49] --> "4->49"
[51,74] --> "51->74"
[76,99] --> "76->99"

Example 2:

**Input:** nums = [], lower = 1, upper = 1
**Output:** ["1"]
**Explanation:** The only missing range is [1,1], which becomes "1".

Example 3:

**Input:** nums = [], lower = -3, upper = -1
**Output:** ["-3->-1"]
**Explanation:** The only missing range is [-3,-1], which becomes "-3->-1".

Example 4:

**Input:** nums = [-1], lower = -1, upper = -1
**Output:** []
**Explanation:** There are no missing ranges since there are no missing numbers.

Example 5:

**Input:** nums = [-1], lower = -2, upper = -1
**Output:** ["-2"]

Constraints:

• -109 <= lower <= upper <= 109

• 0 <= nums.length <= 100

• lower <= nums[i] <= upper

• All the values of nums are unique.

ac

class Solution {
    public List<String> findMissingRanges(int[] nums, int lower, int upper) {
        // two pointers,i & j, for loop j (check nums.length > 2), if [j] == [j-1]+1 continue, else handle
        // left, right, if l == r, add only l. else add l->r.
        // handle first and last.

        List<String> res = new ArrayList<String>();
        int n = nums.length;
        // edge cases
        if (n == 0) {
            addRange(res, lower, upper);
            return res;
        }

        // first one
        if (lower < nums[0]) {
            addRange(res, lower, nums[0]-1);
        }

        // body
        if (n >= 2) {
            for (int i = 1; i < n; i++) {
                long diff = (long)nums[i] - (long)nums[i-1];
                if (diff == 1) continue;
                // stop, check value
                if (diff >= 2) {
                    addRange(res, nums[i-1]+1, nums[i]-1);
                } 
            }
        }

        // last
        if (nums[n-1] < upper) {
            addRange(res, nums[n-1]+1, upper);
        }

        return res;
    }
    private void addRange(List<String> res, int l, int r) {
        if (l == r) {
            res.add(l+"");
        } else if (l < r) {
            res.add(l+"->"+r);
        }
    }
}