[LeetCode] Missing Ranges

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"].

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