Merge Intervals (sort merge)

/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {
    public List<Interval> merge(List<Interval> intervals) {
        if(intervals == null || intervals.size() < 2){
            return intervals;
        }
        Collections.sort(intervals, new Comparator<Interval>(){
            public int compare(Interval i1, Interval i2){
                if(i1.start != i2.start){
                    return i1.start - i2.start;
                } else {
                    return i1.end - i2.end;
                }
            }
        });
        Stack<Interval> st = new Stack<>();
        for (int i=intervals.size()-1; i>=0; --i){
            Interval crt = intervals.get(i);
            while (!st.isEmpty() && crt.end >= st.peek().start){
                Interval top = st.pop();
                crt.end = Math.max(crt.end, top.end);
            }
            st.push(crt);
        }
        return new ArrayList<Interval>(st);
    }
}