leetcode_num148_Sort list，sortlistleetcode

leetcode_num148_Sort list，sortlistleetcode

鏈表的歸併排序

特別注意取中值函數的書寫

/** * 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 *mid=GetMid(head);        ListNode *midnext=mid->next;        mid->next=NULL;        ListNode *lefthead=sortList(head);        ListNode *righthead=sortList(midnext);        return Merge(lefthead,righthead);            }    ListNode *GetMid(ListNode *head){        if(!head||!head->next)            return head;        ListNode *pNode1=head->next;//0,1,2,3 middle 1        ListNode *pNode2=head;        while(pNode1&&pNode1->next){// if pNode1==NULL            pNode1=pNode1->next->next;            pNode2=pNode2->next;        }        return pNode2;    }    ListNode *Merge(ListNode *lefthead,ListNode*righthead){        ListNode *runner= new ListNode(10000);        ListNode *head=lefthead->val<righthead->val?lefthead:righthead;        while(lefthead&&righthead){            if(lefthead->val<righthead->val){                runner->next=lefthead;                lefthead=lefthead->next;                runner=runner->next;            }            else{                runner->next=righthead;                righthead=righthead->next;                runner=runner->next;            }        }        if(lefthead)            runner->next=lefthead;        else if(righthead)            runner->next=righthead;        return head;    }};