题解 | #合并区间#

import java.util.*;

/*
 * public class Interval {
 *   int start;
 *   int end;
 *   public Interval(int start, int end) {
 *     this.start = start;
 *     this.end = end;
 *   }
 * }
 */

/**
 * NC37 合并区间
 * @author d3y1
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param intervals Interval类ArrayList
     * @return Interval类ArrayList
     */
    public ArrayList<Interval> merge (ArrayList<Interval> intervals) {
        // return solution1(intervals);
        // return solution2(intervals);
        return solution3(intervals);
    }

    /**
     * 排序(最小堆)
     * @param intervals
     * @return
     */
    private ArrayList<Interval> solution1(ArrayList<Interval> intervals){
        int n = intervals.size();
        if(n <= 1){
            return intervals;
        }

        PriorityQueue<Interval> minHeap = new PriorityQueue<>((o1, o2) -> (o1.start-o2.start));
        // PriorityQueue<Interval> minHeap = new PriorityQueue<>(new Comparator<Interval>(){
        //     @Override
        //     public int compare(Interval o1, Interval o2){
        //         return o1.start-o2.start;
        //     }
        // });
        for(Interval interval: intervals){
            minHeap.offer(interval);
        }

        ArrayList<Interval> list = new ArrayList<Interval>();
        Interval out = minHeap.poll();
        Interval top;
        while(!minHeap.isEmpty()){
            top = minHeap.poll();
            // 可简化
            // if(out.start<=top.start && top.start<=out.end){
            if(top.start <= out.end){
                out.end = Math.max(out.end, top.end);
            }else{
                list.add(out);
                out = top;
            }
        }
        list.add(out);

        return list;
    }

    /**
     * 排序(数组)
     * @param intervals
     * @return
     */
    private ArrayList<Interval> solution2(ArrayList<Interval> intervals){
        int n = intervals.size();
        if(n <= 1){
            return intervals;
        }

        Collections.sort(intervals, (o1, o2) -> (o1.start-o2.start));

        ArrayList<Interval> list = new ArrayList<Interval>();

        Interval out = intervals.get(0);
        Interval cur;
        for(int i=1; i<n; i++){
            cur = intervals.get(i);
            // 可简化
            // if(out.start<=cur.start && cur.start<=out.end){
            if(cur.start <= out.end){
                out.end = Math.max(out.end, cur.end);
            }else{
                list.add(out);
                out = cur;
            }
        }
        list.add(out);

        return list;
    }

    /**
     * 排序(TreeMap)
     * @param intervals
     * @return
     */
    private ArrayList<Interval> solution3(ArrayList<Interval> intervals){
        int n = intervals.size();
        if(n <= 1){
            return intervals;
        }

        TreeMap<Integer, Integer> startEndMap = new TreeMap<>();
        for(Interval interval: intervals){
            if(startEndMap.containsKey(interval.start)){
                startEndMap.put(interval.start, Math.max(startEndMap.get(interval.start), interval.end));
            }else{
                startEndMap.put(interval.start, interval.end);
            }
        }

        ArrayList<Interval> list = new ArrayList<Interval>();

        Interval cur,out=null;
        for(int start: startEndMap.keySet()){
            cur = new Interval(start, startEndMap.get(start));
            if(out == null){
                out = cur;
            }else if(cur.start <= out.end){
                out.end = Math.max(out.end, cur.end);
            }else{
                list.add(out);
                out = cur;
            }
        }
        list.add(out);

        return list;
    }
}