Missing Ranges

2 cases

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.

func findMissingRanges(nums []int, lower int, upper int) []string {
    nlen := len(nums)
    var res []string

    if nlen == 0 {
        return []string{getMissingRange(lower-1, upper+1)}
    } else if nlen == 1 {
        appendIfNotEmpty(&res, getMissingRange(lower-1, nums[0]))
        appendIfNotEmpty(&res, getMissingRange(nums[0], upper+1))
        return res
    }
    for i := 0; i < nlen; i++ {
        if i == 0 {
            appendIfNotEmpty(&res, getMissingRange(lower-1, nums[i]))
            continue
        }
        appendIfNotEmpty(&res, getMissingRange(nums[i-1], nums[i]))

        if i == nlen-1 {
            appendIfNotEmpty(&res, getMissingRange(nums[i], upper+1))
        }
    }
    return res
}

func getMissingRange(a, b int) string {
    if b-a <= 1 {
        return ""
    } else if b-a == 2 {
        return strconv.Itoa(a + 1)
    } else {
        left, right := strconv.Itoa(a+1), strconv.Itoa(b-1)
        return left + "->" + right
    }
}

func appendIfNotEmpty(list *[]string, s string) {
    if s != "" {
        *list = append(*list, s)
    }
}