[LeetNode]Sort List

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

思路：分治+递归。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
private:
    int getlen(ListNode *head)
    {
        ListNode *p=head;
        int len=0;
        while(p)
        {
            len++;
            p=p->next;
        }
        return len;
    }
    ListNode* merge(ListNode *head1,ListNode *head2)
    {
        ListNode *p=head1,*q=head2,*head=NULL,*cur=NULL;
        while(p&&q)
        {
            if(p->val<=q->val)
            {
                if(head==NULL) head=cur=p;
                else
                {
                    cur->next=p;
                    cur=p;
                }
                p=p->next;
            }
            else
            {
                if(head==NULL) head=cur=q;
                else
                {
                    cur->next=q;
                    cur=q;
                }
                q=q->next;
            }
        }
        if(p) cur->next=p;
        if(q) cur->next=q;
        return head;
    }
public:
    ListNode *sortList(ListNode *head) {
        int len=getlen(head);
        if(len==0||len==1) return head;
        ListNode *p,*q,*r;
        p=r=head;
        int halflen=len>>1;
        for(int i=0;i<halflen-1;i++) r=r->next;
        q=r->next;
        r->next=NULL;
        p=sortList(p);
        q=sortList(q);
        ListNode *newhead=merge(p,q);
        return newhead;
    }
};