[23] leetcode K merge sort list (list, divide and conquer)

Topic links: https://leetcode-cn.com/problems/merge-k-sorted-lists/

Title Description

K merge sort list, return the list sorted combined. Please complexity analysis and description of the algorithm.

Example:

输入:
[
  1->4->5,
  1->3->4,
  2->6
]
输出: 1->1->2->3->4->4->5->6

Thinking

1 one by one twenty-two merging sorted linked list

The combined list of the k problem into two lists combined k-1 times. Here are 21 merge two ordered list of topics.

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode *head = nullptr;
        if(lists.empty()) return head;
        for (auto list:lists) {
            head = mergeTwoLists(head, list);
        }
        return head;
    }

    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;
        if(l1->val < l2->val){
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }else{
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};