class Solution {public:ListNode* sortList(ListNode* head) {if(head == NULL)    return head;Quick_Sort(head, NULL);return head;    }void Quick_Sort(ListNode* head, ListNode* end){if (head != end && head->next != end){ListNode* ptr = head;while(ptr->next != end) ptr = ptr->next; // 取链表最后一个元素作为主元int pivot = ptr->val;ListNode* p1 = head;ListNode* p2 = head;while(p2->next != end){if(p2->val < pivot){int temp = p2->val;p2->val = p1->val;p1->val = temp;p1 = p1->next;}p2 = p2->next;}p2->val = p1->val;p1->val = pivot;Quick_Sort(head, p1); // 此时 p1 指向 pivot，前一半链表头为 head，尾为 p1Quick_Sort(p1->next, end); // 后一半链表头为 p1->next，尾为 end}}

class Solution {public:ListNode* sortList(ListNode* head) {return  Merge_Sort(head);}ListNode* Merge_Sort(ListNode* head){ListNode * mid = Find_Mid(head);if (mid){ListNode* right_head = mid->next;mid->next = NULL;ListNode* left_head = Merge_Sort(head);right_head = Merge_Sort(right_head);head = Merge_List(left_head, right_head);}return head;}ListNode* Merge_List(ListNode* left_head, ListNode* right_head){ListNode* l1 = left_head;ListNode* l2 = right_head;if (left_head->val > right_head->val){l1 = right_head;l2 = left_head;}// l1 指向第一个节点值较小的链表，然后将 l2 中的每个节点插入到 l1 中ListNode* p1 = NULL;ListNode* head = l1;while (l1->next && l2){if (l1->next->val > l2->val){p1 = l1->next;l1->next = l2;l2 = l2->next;l1 = l1->next;l1->next = p1;}else{l1 = l1->next;}}if (l2) l1->next = l2; //若 l2 还有节点直接插入到 l1 后面即可return head;}ListNode* Find_Mid(ListNode* head){if (head == NULL || head->next == NULL) return NULL;ListNode* slow = head;ListNode* fast = head;while (fast->next != NULL && fast->next->next != NULL){slow = slow->next;fast = fast->next->next;}return slow;}};