题解 | #合并k个已排序的链表#

import java.util.*;
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        if(lists == null || lists.size() < 1) return null;
        return mergeHelper(lists,0,lists.size() - 1);
    }
    
    private ListNode mergeHelper(ArrayList<ListNode> lists,int left,int right){
        if(left > right ) return null;
        if(left == right) return lists.get(left);
        int mid = left + (right - left)/2;
        return merge(mergeHelper(lists,left,mid),mergeHelper(lists,mid+1,right));
    }
    
    // 合并两个有序链表!
    private ListNode merge(ListNode l1,ListNode l2){
        ListNode fake_head = new ListNode(-1);
        ListNode pre = fake_head;
        ListNode cur1 = l1,cur2 = l2;
        
        while(cur1 != null && cur2 != null){
            if(cur1.val < cur2.val){
                pre.next = cur1;
                cur1 = cur1.next;
            }else {
                pre.next = cur2;
                cur2 = cur2.next;
            }
            pre = pre.next;
        }
        
        if(cur1 != null){
            pre.next = cur1;
        }
        if(cur2 != null){
            pre.next = cur2;
        }
        return fake_head.next;
    }
}