FB面经Prepare: Merge K sorted Array

Posted 2020-10-13

跟Merge K sorted lists不同在于，从PQ里poll出来以后不知道下一个需要被加入PQ的是哪一个

所以需要写一个wrapper class

import java.util.*;

public class MergeKLists {
    static class Pair {
        int listIndex;
        int idInList;
        int value;
        public Pair(int l, int id, int val) {
            this.listIndex = l;
            this.idInList = id;
            this.value = val;
        }
    }
    
    public static List<Integer> merge(List<List<Integer>> lists) {
        List<Integer> res = new ArrayList<Integer>();
        
        Comparator<Pair> comp = new Comparator<Pair>() {
            public int compare(Pair p1, Pair p2) {
                return p1.value - p2.value;
            }
        };
        PriorityQueue<Pair> pq = new PriorityQueue<Pair>(1, comp);
        for (int i=0; i<lists.size(); i++) {
            Pair p = new Pair(i, 0, lists.get(i).get(0));
            pq.offer(p);
        }
        
        while (!pq.isEmpty()) {
            Pair pa = pq.poll();
            int index = pa.listIndex;
            int id = pa.idInList;
            if (id < lists.get(index).size()-1) {
                pq.offer(new Pair(index, id+1, lists.get(index).get(id+1)));
            }
            res.add(pa.value);
        }
        
        return res;
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        List<Integer> l1 = Arrays.asList(1,2,2,3,6);
        List<Integer> l2 = Arrays.asList(1,4,5,7,8,9);
        List<Integer> l3 = Arrays.asList(3,3,3,5,10);
        List<List<Integer>> lists = new ArrayList<List<Integer>>();
        lists.add(new ArrayList<Integer>(l1));
        lists.add(new ArrayList<Integer>(l2));
        lists.add(new ArrayList<Integer>(l3));
        List<Integer> res = merge(lists);
        System.out.println(res);
    }

}