[LeetCode] Missing Ranges

Problem Description:

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"].

---

Well, I guess this problem aims at testing your ability to think throughly. The tricky part in this problem is to take all possible cases into consideration and unify them to give a succinct code.

The code is as follows.

class Solution {
public:
    vector<string> findMissingRanges(vector<int>& nums, int lower, int upper) {
        vector<string> ranges;
        int pre = lower - 1, n = nums.size();
        for (int i = 0; i <= n; i++) {
            int cur = (i == n) ? upper + 1 : nums[i];
            if (cur - pre >= 2)
                ranges.push_back(missingRanges(pre + 1, cur - 1));
            pre = cur;
        }
        return ranges;
    }
private:
    string missingRanges(int low, int up) {
        return (low == up) ? to_string(low) : to_string(low) + "->" + to_string(up);
    }
};