Merge k Sorted Lists

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

Solution: Find the smallest list-head first using minimum-heap(lgk).
complexity: O(NlgK)

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     class compare {
12     public:
13         bool operator()(ListNode *a, ListNode *b) {
14             return a->val > b->val; // minimum-heap
15         }
16     };
17     
18     ListNode *mergeKLists(vector<ListNode *> &lists) {
19         priority_queue<ListNode*, vector<ListNode*>, compare> q;
20         for(int i = 0; i < lists.size(); i++) {
21             if(lists[i]) {
22                 q.push(lists[i]);
23             }
24         }
25         ListNode dummy(0), *cur = &dummy;
26         while(!q.empty()) {
27             ListNode* node = q.top(); q.pop();
28             cur->next = node;
29             cur = cur->next;
30             if(node->next) {
31                 q.push(node->next);
32             }
33         }
34         return dummy.next;
35     }
36 };