Merge Intervals [Leetcode]

Given a collection of intervals, merge all overlapping intervals.
For example,
Given [1,3],[2,6],[8,10],[15,18],
return [1,6],[8,10],[15,18].

Solution: sort the input intervals in term of starts first, then scan the intervals and merge overlapped intervals. Note: Leetcode prohibit passing the function pointer as the third argument of sort routine. The function objector is the unique choice.

/**
 * Definition for an interval.
 * struct Interval {
 *     int start;
 *     int end;
 *     Interval() : start(0), end(0) {}
 *     Interval(int s, int e) : start(s), end(e) {}
 * };
 */
class Solution {
public:
    class compare{
    public:
        bool operator()(Interval a, Interval b){
            return a.start<b.start;
        }
    };
    vector<Interval> merge(vector<Interval> &intervals) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<Interval> ret;
        if(intervals.empty()) return ret;
        //sort the intervals
        sort(intervals.begin(), intervals.end(), compare());
        ret.push_back(intervals[0]);
        for(int i=1; i<intervals.size(); i++){
            if(intervals[i].start>ret.back().end)
                ret.push_back(intervals[i]);
            else
                ret.back().end = max(ret.back().end, intervals[i].end);
        }
        return ret;
    }
};