19.1.29 [LeetCode 23] Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

struct cmp {
    bool operator () (ListNode *a, ListNode *b) {
        return a->val > b->val;
    }
};

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode*ans=new ListNode(0);
        ListNode*head = ans;
        priority_queue<ListNode*, vector<ListNode*>, cmp>q;
        for (int i = 0; i < lists.size(); i++)
            if(lists[i])
                q.push(lists[i]);
        while (!q.empty()) {
            ListNode* tmp = q.top();
            ans->next = tmp;
            q.pop();
            if(tmp->next)
                q.push(tmp->next);
            ans = ans->next;
        }
        return head->next;
    }
};

看到马上想到的是赢者树败者树，但是太麻烦了