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
10 struct helper {
11 ListNode *head;
12 int len;
13 helper(ListNode *h, int l) : head ( h ), len ( l ) {}
14 };
15
16 class helpercmp {
17 public:
18 bool operator() (const helper &a, const helper &b) {
19 return a.len > b.len;
20 }
21 };
22
23 class Solution {
24 public:
25 priority_queue<helper, vector<helper>, helpercmp> heap;
26
27 inline int listSize(ListNode *head) {
28 int len = 0;
29 for ( ; head != nullptr; head = head->next, len++ );
30 return len;
31 }
32 ListNode *mergeKLists(vector<ListNode *> &lists) {
33 if ( lists.empty() ) return nullptr;
34 if ( lists.size() == 1 ) return lists[0];
35 ListNode *head = nullptr, *left = nullptr, *right = nullptr;
36 for ( auto list : lists ) {
37 heap.push( helper(list, listSize(list) ) );
38 }
39 while ( heap.size() != 1 ) {
40 left = heap.top().head;
41 heap.pop();
42 right = heap.top().head;
43 heap.pop();
44 head = mergeList( left, right );
45 heap.push( helper( head, listSize(head) ) );
46 }
47 return heap.top().head;
48 }
49 ListNode *mergeList(ListNode *a, ListNode *b) {
50 ListNode dummy(0);
51 ListNode *tail = &dummy;
52 while ( a != nullptr && b != nullptr ) {
53 if ( a-> val <= b->val ) {
54 tail->next = a;
55 a = a->next;
56 } else {
57 tail->next = b;
58 b = b->next;
59 }
60 tail = tail->next;
61 }
62 tail->next = a == nullptr ? b : a;
63 return dummy.next;
64 }
65 };
