leetcode[148]Sort List

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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
ListNode *merge(ListNode *left,ListNode *right)
{
    ListNode *newl=new ListNode(0);
    ListNode *p=newl;
    while(left&&right)
    {
        if(left->val<=right->val)
        {
           p->next=left;
           left=left->next;
        }
        else
        {
            p->next=right;
            right=right->next;
        }
        p=p->next;
    }
    if(left)p->next=left;
    if(right)p->next=right;
    return newl->next;
}
ListNode *mergeSort(ListNode *head)
{
    if(head==NULL||head->next==NULL)return head;
    ListNode *slow=head;
    ListNode *fast=head;
    ListNode *pre=NULL;
    while(fast&&fast->next)
    {
        fast=fast->next->next;
        pre=slow;
        slow=slow->next;
    }
    pre->next=NULL;
    ListNode *left=mergeSort(head);
    ListNode *right=mergeSort(slow);
    return merge(left,right);
}
    ListNode *sortList(ListNode *head) {
        if(head==NULL||head->next==NULL)return head;
        return mergeSort(head);
    }
};