Java菜鸟入门(21) Merge K sorted List

假设输入是K个iterator, 如何来merge这K个iterator of Integer,使得输出是 List?

1. 类似Merge K sorted List普通题的思路，使用PriorityQueue(or, Heap

2. 使用一个wrapper来把current Integer value和它的相关的Iterator给捆绑打包。这样做的原因是，每次使用了iterator.next()了，也就是当前Heap中删除了一个数字需要补充比它大的一个数了，如果不记录iterator,就没有办法为这个新数字调用iterator.hasNext()和接下来的iterator.next

import java.util.*;
public class Solution {
    private Comparator myComparator = new Comparator() {
        public int compare(IteratorWrapper left, IteratorWrapper right) {
            return left.val - right.val;
        }
    };

    public class IteratorWrapper{
        Integer val;
        Iterator it;
        public IteratorWrapper(Integer v, Iterator iter) {
            val= v;
            it = iter;
        }
    }

    public List mergeKLists(List> lists) {
        if (lists == null || lists.size() == 0) {
            return null;
        }
        PriorityQueue heap = new PriorityQueue(lists.size(), myComparator);
        for (int i =0; i < lists.size(); ++i) {
            if (lists.get(i) != null && lists.get(i).hasNext()) {
                heap.offer(new IteratorWrapper(lists.get(i).next(), lists.get(i)));
            }
        }
        List result = new ArrayList();
        while (!heap.isEmpty()) {
            IteratorWrapper iw = heap.poll();
            result.add(iw.val);
            if (iw.it.hasNext()) {
                iw.val = iw.it.next();
                heap.offer(iw);
            }
        }
        return result;
    }
}

测试用的main函数和输出如下:

    public static void main (String [] args) {
        List ll1 = new ArrayList<>();
        ll1.add(1);
        ll1.add(3);
        ll1.add(5);
        ll1.add(7);
        ll1.add(10);
        ll1.add(30);
        ll1.add(90);
        List ll2 = new ArrayList<>();
        ll2.add(2);
        ll2.add(3);
        ll2.add(6);
        ll2.add(17);
        ll2.add(50);
        ll2.add(130);
        ll2.add(150);
        List ll3 = new ArrayList<>();
        ll3.add(35);
        ll3.add(49);
        List> listIterators = new ArrayList> ();
        listIterators.add(ll3.iterator());
        listIterators.add(ll2.iterator());
        listIterators.add(ll1.iterator());
        Solution s = new Solution();
        List result = s.mergeKLists(listIterators);
        System.out.println(result.toString());
        //output [1, 2, 3, 3, 5, 6, 7, 10, 17, 30, 35, 49, 50, 90, 130, 150]
    }