單向連結清單的插入排序和歸并排序

插入排序: O(n^2)

class Solution {
public:
    ListNode* insertionSortList(ListNode* head) {
        ListNode *dummy = new ListNode(-1), *cur = dummy;
        while (head) {
            ListNode* tmp = head->next;
            cur = dummy;
            while (cur->next && cur->next->val <= head->val) {
                cur = cur->next;
            }
            head->next = cur->next;
            cur->next = head;
            head = tmp;
        }
        ListNode* result = dummy->next;
        delete dummy;
        return result->next;
    }
};
           

歸并排序：O(nlogn)

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (!head || !head->next)
            return head;
        ListNode *slow = head, *fast = head, *pre = head;
        while (fast && fast->next) {
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        pre->next = NULL;
        return merge(sortList(head), sortList(slow));
    }
    
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode *dummy = new ListNode(INT_MIN);
        ListNode *cur = dummy;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                cur->next = l1;
                l1 = l1->next;
            }
            else {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        if (l1)
            cur->next = l1;
        if (l2)
            cur->next = l2;
        ListNode *result = dummy->next;
        delete dummy;
        return result;
    }
    
};