Merge k Sorted Lists(合并k个排序链表)

问题

Merge k sorted linked lists and return it as one sorted list.

Analyze and describe its complexity.

Have you met this question in a real interview? Yes
Example
Given lists:

[
2->4->null,
null,
-1->null
],
return -1->2->4->null.

分析

递归分治再加上归并排序。

代码

 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param lists: a list of ListNode
     * @return: The head of one sorted list.
     */
    public ListNode mergeKLists(List lists) {  
        // write your code here
        if(lists==null||lists.isEmpty()){
            return null;
        }
        return merge(lists,0,lists.size()-1);
    }
    private ListNode merge(List lists,int l,int r){
        if(l==r){
            return lists.get(l);
        }
        int mid=l+(r-l)/2;
        ListNode left=merge(lists,l,mid);
        ListNode right=merge(lists,mid+1,r);
        return realMerge(left,right);
    }
    private ListNode realMerge(ListNode l,ListNode r){
        ListNode head=new ListNode(0);
        ListNode temp=head;
        while(true){
            if(l==null){
                temp.next=r;
                break;
            }
            if(r==null){
                temp.next=l;
                break;
            }
            if(l.val>r.val){
                temp.next=r;
                temp=temp.next;
                r=r.next;
            }else{
                temp.next=l;
                temp=temp.next;
                l=l.next;
            }
        }
        return head.next;
    }
}