一、插入排序（InsertionSortList）
单链表的插入排序，折腾了好久一会儿，好不容易做出来，在leetcode上仅仅击败百分之十几的人，而且代码啰嗦，参考了下别人代码，提交了下面的代码接近50%
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 == nullptr) return head; 13 if(head->next == nullptr) return head; 14 15 ListNode* helper = new ListNode(-1); 16 17 ListNode* cur= head; 18 ListNode* pre; 19 ListNode* next; 20 21 while(cur){ 22 next = cur->next; 23 pre = helper; 24 25 while(pre->next != nullptr && pre->next->val < cur->val){ 26 pre = pre -> next; 27 } 28 29 cur->next = pre->next; 30 pre->next = cur; 31 cur = next; 32 } 33 34 head = helper->next; 35 delete helper; 36 return head; 37 } 38 };
二、归并排序（相比上述的InsertionSortList,单链表的归并排序，时间复杂度为nlog(n)，但是小数据两的排序，插入排序更好
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) return head; 13 14 ListNode* fast = head; 15 ListNode* slow = head; 16 while(fast->next != nullptr && fast->next->next != nullptr){ 17 fast = fast->next->next; 18 slow = slow->next; 19 } 20 fast = slow; 21 slow = slow->next; 22 fast->next = nullptr; 23 24 ListNode* L1 = sortList(head); 25 ListNode* L2 = sortList(slow); 26 27 return mergeTwoLists(L1,L2); 28 } 29 private: 30 ListNode* mergeTwoLists(ListNode* l1,ListNode* l2){ 31 if(l1 == nullptr) return l2; 32 if(l2 == nullptr) return l1; 33 34 ListNode* head = new ListNode(-1); 35 ListNode* p = head; 36 while(l1 && l2) 37 { 38 if(l1->val < l2->val){ 39 p->next = l1; 40 l1 = l1-> next; 41 p = p->next; 42 }else{ 43 p->next = l2; 44 l2 = l2->next; 45 p = p->next; 46 } 47 } 48 if(l1) p->next = l1; 49 else p->next = l2; 50 p = head->next; 51 delete head; 52 return p; 53 } 54 };