Given a sorted integer array without duplicates, return the summary of its ranges.

For example, given [0,1,2,4,5,7], return ["0->2","4->5","7"].

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

class Solution {
public:
    vector<string> summaryRanges(vector<int>& nums) {
        vector<string> ans;
        string range;
        if(nums.size() == 0) return ans;
        int prev = nums[0];
        int start = nums[0];
        //nums.push_back(0x80000000);//sentinel BUG HERE, MAY OVERFLOW
        for(int i = 1; i < nums.size(); i++){
            int num = nums[i];
            if(num - prev == 1){
                //continual
                prev = num;
            }
            else{
                //interval
                if(start == prev){
                    //single number
                    range.clear();
                    range.append(to_string(start));
                    ans.push_back(range);
                }
                else{
                    //range numbers
                    range.clear();
                    range.append(to_string(start));
                    range.append("->");
                    range.append(to_string(prev));
                    ans.push_back(range);
                }
                start = prev = num;
            }
        }
        if(start == prev){
                    //single number
                    range.clear();
                    range.append(to_string(start));
                    ans.push_back(range);
                }
                else{
                    //range numbers
                    range.clear();
                    range.append(to_string(start));
                    range.append("->");
                    range.append(to_string(prev));
                    ans.push_back(range);
                }
        return ans;
    }
};