Sort List & Insertion Sort List
148. Sort List
题目链接：https://leetcode.com/problems/sort-list/#/description
题目大意：给定一个链表，要求以O(nlogn)时间复杂度和O(1)空间复杂度将链表排序。
思路：使用归并排序
算法步骤：（1）使用快慢指针将链表分为两部分，如果链表长度小于2，则不划分，直接返回链表；（2）将得到的两个链表分别排序；（3）合并两个有序链表。
算法复杂度：时间复杂度O(nlog(n))，空间复杂度O(1)
代码：
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 || !head->next) 13 return head; 14 ListNode *fast = head, *slow = head; 15 while (fast->next && fast->next->next) { 16 fast = fast->next->next; 17 slow = slow->next; 18 } 19 fast = slow->next; 20 slow->next = nullptr; 21 22 ListNode *l1 = sortList(head); 23 ListNode *l2 = sortList(fast); 24 25 return mergeList(l1, l2); 26 } 27 ListNode *mergeList(ListNode *l1, ListNode *l2) { 28 ListNode res(0), *pres = &res; 29 while (l1 && l2) { 30 if (l1->val <= l2->val) { 31 pres->next = l1; 32 l1 = l1->next; 33 } 34 else { 35 pres->next = l2; 36 l2 = l2->next; 37 } 38 pres = pres->next; 39 } 40 pres->next = l1 ? l1 : l2; 41 return res.next; 42 } 43 };
评测系统上运行结果：
147. Insertion Sort List
题目链接：https://leetcode.com/problems/insertion-sort-list/#/description
题目大意：给定一个链表，使用插入排序算法对链表进行排序
算法步骤：（1）while循环找到插入点；（2）将当前节点插入插入点。
算法复杂度：时间复杂度O(n^2)，空间复杂度O(1)
代码：
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* insertionSortList(ListNode* head) { 12 if (!head || !head->next) 13 return head; 14 ListNode pstart(0), *pcurr = head->next, *ppre = head; 15 pstart.next = head; 16 while(pcurr) { 17 ListNode *p = &pstart; 18 while (p->next != pcurr && p->next->val <= pcurr->val) 19 p = p->next; 20 if (p->next == pcurr) { 21 ppre = ppre->next; 22 pcurr = pcurr->next; 23 } else { 24 ppre->next = pcurr->next; 25 pcurr->next = p->next; 26 p->next = pcurr; 27 pcurr = ppre->next; 28 } 29 } 30 return pstart.next; 31 } 32 };
评测系统上运行结果：
