（链表）链表的排序问题

/**
 * 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) {
        vector<int> res;
        ListNode *temp = head;
        int length = 0;
        while (temp){
            res.push_back(temp->val);
            temp = temp->next, length++;
        }
        sort(res.begin(), res.end());
        temp = head;
        int i =0;
        while (temp && i<length){
            temp->val = res[i];
            temp = temp->next;
            i++;
        }
        return head;
    }
};

/**
 * 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* dummyhead = new ListNode(0);
        ListNode* pre = dummyhead;
        ListNode* cur = head;  

        while(cur != NULL)
        {
            ListNode* next = cur->next;  //维护链表的下一个结点
            pre = dummyhead;             //重置pre为新链表头开始  

            //在当前排好的新链表中找到第一个大于cur->val的结点
            while(pre->next != NULL && pre->next->val <= cur->val)
            {
                pre = pre->next;
            }  

            //当前pre的next结点的值大于cur的值,将cur插入到pre后
            cur->next = pre->next;
            pre->next = cur;
            cur = next;   //cur指向原链表的下一个节点
        }
        return dummyhead->next;
    }
};