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() == ) res.push_back(printRange(lower, upper));
         else if (nums.front() > lower) res.push_back(printRange(lower, nums.front() - ));
         for (int i = , n = nums.size(); i < n; i++)
             if (nums[i] > nums[i-] + )
                 res.push_back(printRange(nums[i-] + , nums[i] - ));
         if (nums.size() >  && nums.back() < upper)
             res.push_back(printRange(nums.back() + , upper));
         return res;
     }
 };