148. Sort List

题目：

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

解题思路：

使用对链表的归并排序。

/**
 * 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==NULL || head->next==NULL)return head;
        ListNode *slow = head, *fast = head->next; //一开始是这样的，报错。细思！！！(ListNode* slow = head, fast = head->next;)
        while(fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* head2 = slow->next;
        slow->next = NULL;
        ListNode* l = sortList(head);
        ListNode* r = sortList(head2);
        return merge(l,r);
    }
    
    ListNode* merge(ListNode* left, ListNode* right){
        ListNode *res = new ListNode(0), *node; //一开始是这样的，报错。细思！！！(ListNode* res(0), node;)
        node = res;
        while(left && right)
        {
            if(left->val<=right->val)
            {
                node->next = left;
                node = node->next;
                left = left->next;
            }
            else
            {
                node->next = right;
                node = node->next;
                right = right->next;
            }
        }
        if(left)node->next = left;
        if(right)node->next = right;
        return res->next;
    } 
};