【Leetcode】Sort List

Sort a linked list in O(n log n) time using constant space complexity.

单向链表排序O(nlogn)，Mergesort可以实现。

 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     ListNode *sortList(ListNode *head) {
12         if (head == nullptr || head->next == nullptr) {
13             return head;
14         }
15         //快慢指针找中间节点
16         ListNode *fast = head, *slow = head;
17         while (fast->next != nullptr && fast->next->next != nullptr) {
18             fast = fast->next->next;
19             slow = slow->next;
20         }
21 
22         fast = slow->next;
23         slow->next = nullptr;
24         ListNode *l1 = sortList(head);
25         ListNode *l2 = sortList(fast);
26         return mergeTwoLists(l1, l2);
27     }
28 
29 private:
30     // Merge Two Sorted Lists
31     ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
32         ListNode dummy(-1);
33         for (ListNode* p = &dummy; l1 != nullptr || l2 != nullptr; p = p->next) {
34             int val1 = l1 == nullptr ? INT_MAX : l1->val;
35             int val2 = l2 == nullptr ? INT_MAX : l2->val;
36             if (val1 <= val2) {
37                 p->next = l1;
38                 l1 = l1->next;
39             } else {
40                 p->next = l2;
41                 l2 = l2->next;
42             }
43         }
44         return dummy.next;
45     }
46 };