23 Merge k Sorted Lists

Leetcode

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:

[
  1->4->5,
  1->3->4,
  2->6
]

merging them into one sorted list:

1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

可以使用类似 Merge Sort 的方式归并。

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists==null || lists.length<1) return null;
        return mergeKLists(lists, lists.length);
    }

    //num 表示剩余未 merge 的 List ，仅剩 1 个的时候返回
    private ListNode mergeKLists(ListNode[] lists, int num) {
        if(num==1) return lists[0];
        int index=0;
        for(int i=0;i<num-1;i=i+2){
            lists[index++] = merge2(lists[i], lists[i+1]);
        }
        if(num%2==1){
            lists[index++]=lists[num-1];
        }
        return mergeKLists(lists, index);
    }

    //归并2个 List
    private ListNode merge2(ListNode l1, ListNode l2){
        ListNode head = new ListNode();
        ListNode t = head;
        while(l1!=null && l2!=null){
            if(l1.val<l2.val){
                t.next=l1;
                l1=l1.next;
            } else {
                t.next=l2;
                l2=l2.next;
            }
            t=t.next;
        }
        if(l1==null) {
            t.next=l2;
        } else {
            t.next=l1;
        }
        return head.next;
    }
}