LeetCode(M) Sort List

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

根据题目O(nlogn)时间，O(1)空间，选择了归并排序

/** * 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) return NULL;       if(!head->next) return head;       // ListNode*pre=new ListNode(-1);       /* pre->next=head;        ListNode *fast=pre->next,*slow=pre->next->next;        while(slow->next&&slow->next->next){            fast=fast->next;            slow=slow->next->next;        }        fast->next=NULL;*/        ListNode *slow = head, *fast = head, *pre = head;         while (fast && fast->next) {            pre = slow;            slow = slow->next;            fast = fast->next->next;        }        pre->next = NULL;    //   sortList(fast);sortList(slow);     //   return  merge(fast,slow);   //   sortList(head);sortList(slow);   //  return merge(head,slow);     return merge(sortList(head), sortList(slow));    }    ListNode* merge(ListNode* first,ListNode *second){        ListNode *temp=new ListNode(-1),*p=first,*q=second,*pre;        pre=temp;        while(p&&q){        if(p->val>q->val) {temp->next=q;q=q->next;}        else{temp->next=p;p=p->next;}        temp=temp->next;        }        if(p) temp->next=p;//这里由while改成了if       if(q) temp->next=q;        return pre->next;    }    };

打了注释的地方都是错误点

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