链表排序-leetcode#148

/**
 * 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 * slow = head;
        ListNode * fast = head;
        ListNode * pre = head;
        while(fast && fast->next){
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        pre->next =NULL;
        return mergeList(sortList(head),sortList(slow));//合并有序的两个链表
    }
    ListNode * mergeList(ListNode* p1,ListNode * p2){
        if(!p1) return p2;
        if(!p2) return p1;
        if(p1->val < p2->val){
            p1->next = mergeList(p1->next,p2);
            return p1;
        }else{
            p2->next = mergeList(p1,p2->next);
            return p2;
        }
    }
};