leetcode 148. 排序链表(c++)

Posted 2022-06-18 xiaotongtt

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4
示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

/**
 * Definition for singly-linked list.
 * struct ListNode 
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) 
 * ;
 */
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(-1);
        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;
        return dummy->next;
    
;