leetcode4:sort-list

题目描述

在O(n log n)的时间内使用常数级空间复杂度对链表进行排序。

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

示例1

输入

{3,2,4}

输出

复制

{2,3,4}

class Solution {
public:
    ListNode* findMiddle(ListNode* head){
        ListNode* chaser = head;
        ListNode* runner = head->next;
        while(runner != NULL && runner->next != NULL){
            chaser = chaser->next;
            runner = runner->next->next;
        }
        return chaser;
    }
     
 ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == NULL){
            return l2;
        }
        if(l2 == NULL){
            return l1;
        }
        ListNode* dummy = new ListNode(0);
        ListNode* head = dummy;
        while(l1 != NULL && l2 != NULL){
            if(l1->val > l2->val){
                head->next = l2;
                l2 = l2->next;
            }
            else{
                head->next = l1;
                l1 = l1->next;
            }
            head = head->next;
        }
        if(l1 == NULL){
            head ->next = l2;
        }
        if(l2 == NULL){
            head->next = l1;
        }
        return dummy->next;
    }
     
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head ->next == NULL){
            return head;
        }
        ListNode* middle = findMiddle(head);
        ListNode* right = sortList(middle->next);
        middle -> next = NULL;
        ListNode* left = sortList(head);
        return mergeTwoLists(left, right);
    }
};