23. 合并K个排序链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
//优先队列
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists.length==0)
            return null;
        Queue<ListNode> pq=new PriorityQueue(lists.length,new Comparator<ListNode>()
                                                {
                                                    @Override
                                                    public int compare(ListNode o1,ListNode o2)
                                                    {
                                                        return o1.val-o2.val;
                                                    }
                                                });
        ListNode root=new ListNode(0);
        for(int i=0;i<lists.length;i++)
            if(lists[i]!=null)
                pq.add(lists[i]);
        ListNode p=root;
        while(!pq.isEmpty())
        {
            ListNode q=pq.remove();
            p.next=q;
            p=p.next;
            if(q.next!=null)
            pq.add(q.next);
        }
        return root.next;
    }
}

//分治法
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists.length==0)
            return null;
        return mergeLists(lists,0,lists.length-1);
    }
    public ListNode mergeLists(ListNode[] lists,int l,int r)
    {
        if(l==r)
            return lists[l];
        int mid=l+(r-l)/2;
        ListNode l1=mergeLists(lists,l,mid);
        ListNode l2=mergeLists(lists,mid+1,r);
        return merge(l1,l2);
    }
    public ListNode merge(ListNode l1,ListNode l2)
    {
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
        if(l1.val<l2.val)
        {
            l1.next=merge(l1.next,l2);
            return l1;
        }
        else
        {
            l2.next=merge(l1,l2.next);
            return l2;
        }
    }
}