Summary Ranges

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"].

class Solution {
public:
    string format(int start,int end)
    {
        char buf[40];
        if(start==end){
            sprintf(buf,"%d",start);
        }else{
            sprintf(buf,"%d->%d",start,end);
        }
        return string(buf);
    }
    vector<string> summaryRanges(vector<int>& nums) {
        int numsSize = nums.size();
        int start = -1;
        int end = -1;
        vector<string> res;
        for(int i=0;i<numsSize;i++){
            if(i==0){
                start = nums[i];
            }else{
                if(nums[i]!=nums[i-1]+1){
                    end = nums[i-1];
                    res.push_back(format(start,end));
                    start = nums[i];    
                }
            }
            if(i==numsSize-1){
                end = nums[i];
                res.push_back(format(start,end));
            }
        }
        return res;
    }
};