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

复制

{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);

    }

};