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"].

class Solution {
public:
    string printRange(int lower, int upper) {
        string low = std::to_string(lower);
        string up = std::to_string(upper);
        if (lower == upper) return low;
        return low + "->" + up;
    }
    vector<string> findMissingRanges(vector<int>& nums, int lower, int upper) {
        vector<string> res;
        if (nums.size() == 0) res.push_back(printRange(lower, upper));
        else if (nums.front() > lower) res.push_back(printRange(lower, nums.front() - 1));
        for (int i = 1, n = nums.size(); i < n; i++)
            if (nums[i] > nums[i-1] + 1)
                res.push_back(printRange(nums[i-1] + 1, nums[i] - 1));
        if (nums.size() > 0 && nums.back() < upper)
            res.push_back(printRange(nums.back() + 1, upper));
        return res;
    }
};