148. Sort List

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

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return head;
        // 将链表分成两个
        ListNode* pre = nullptr, *slow = head, *fast = head;
        while(fast != nullptr && fast->next != nullptr)
        {
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        pre->next = nullptr;
        // 分而治之
        ListNode* l1 = sortList(head);
        ListNode* l2 = sortList(slow);
        // 合并
        return merge(l1, l2);
    }
    
    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        ListNode* p = nullptr;
        if(l1->val < l2->val)
        {
            p = l1;
            l1 = l1->next;
        }
        else
        {
            p = l2;
            l2 = l2->next;
        }
        ListNode* l = p;
        
        while(l1 != nullptr && l2 != nullptr)
        {
            if(l1->val < l2->val)
            {
                p->next = l1;
                l1 = l1->next;
            }
            else
            {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        if (l1 != nullptr)
            p->next = l1;
        if (l2 != nullptr)
            p->next = l2;
        return l;
    }
};