LeetcodeOJ: Merge k Sorted Lists 归并排序+最小堆

 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 };