LeetCode/148.Sort List at master · xxkafu/LeetCode · GitHub

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//solution:
//ÓÃmergesort
/**
 * 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) {
        // determine the num
        ListNode* tmp=head;
        int listSize=0;
        while(tmp!=NULL){
            tmp=tmp->next;
            listSize++;
        }
        // determine the loop time;
        int halfLen= listSize/2;
        if(listSize%2)
            halfLen++;
        int mergeLen=1;
        while(mergeLen<halfLen){
            mergeLen *=2;
        }
        ListNode* nullhead = new ListNode(-1);
        nullhead->next = head;

        for(int i=1; i<=mergeLen; i=i*2){
            ListNode* l1head=nullhead->next;// the merge first list
            ListNode* l2head=nullhead->next;// the merge second list
            ListNode* oldend = nullhead;
            while(l1head!=NULL){
                ListNode* preL1=l1head;
                ListNode* itrL1=preL1->next;
                for(int j=1;j<i&&itrL1!=NULL;j++){
                    preL1=preL1->next;
                    itrL1 = itrL1->next;
                }// find the l2head, l1end

                l2head= itrL1;// l2 head find
                if(l2head==NULL){
                    oldend->next = l1head;
                    break;
                }
                preL1->next=NULL;// l1 list formed

                ListNode* preL2= l2head;
                ListNode* itrL2=preL2->next;
                for(int j=1;j<i&&itrL2!=NULL;j++){
                    preL2=preL2->next;
                    itrL2 = itrL2->next;
                }// find the l2end ,and the new l1head

                preL2->next=NULL;// l2 list formed
                ListNode* newhead;
                ListNode* newend;
                mergeTwoLists(l1head,l2head,newhead,newend);
                l1head=itrL2;// the l1 head updata
                oldend->next=newhead;
                oldend = newend;
            }
        }

        return nullhead->next;
    }

        void mergeTwoLists(ListNode *l1, ListNode *l2,ListNode* &newhead,ListNode* &newend) {

        if(l1->val>l2->val) {
            swap(l1,l2);
        }

        ListNode *retList= l1;
        while(l1->next!=NULL) {
            if(l1->next->val<=l2->val) {
                l1= l1->next;
            }
            else {
                ListNode *tmp = l1->next;
                l1->next = l2;
                l2= tmp;
                l1= l1->next;
            }
        }
            l1->next = l2;
            while(l2->next!=NULL)
                l2=l2->next;
            newhead = retList;
            newend = l2;
    }
};