sort-list

说说自己做的过程中的几个错误吧。
1忽略了空链表的情况。
2两个链表进行并归排序的时候，应该拿函数返回的头结点，而不是划分的头结点，因为在递归的过程中，最初的头结点可能因为排序而后移了。
3快慢指针的问题条件的判断。

#include <iostream>
#include <map>
#include <vector>
#include<cstdlib>
#include <ctime>
#include <fstream>
using namespace std;

  struct ListNode {
     int val;
      ListNode *next;
      ListNode(int x) : val(x), next(NULL) {}
  };
    ListNode *sortList(ListNode *head) {
        ListNode* pf = head,*ps = head;
        if(!head) return 0;
        while(pf->next&&pf->next->next) {
            pf=pf->next->next;
            ps=ps->next;
        }
        //pf->val ps->val head->val ans->val
        pf= ps->next; ps->next=0;
        ListNode* head1=head,*head2=pf;
        if(head&&head->next) head1=sortList(head);
        if(pf&&pf->next)     head2=sortList(pf);
             ListNode* ans = new ListNode(-1);
            ListNode* ans1 = ans;
        while(head1&&head2) {
            if(head1->val<=head2->val) {ans->next=head1;head1=head1->next;}
            else if(head1->val>head2->val) {ans->next=head2;head2=head2->next;}
            ans = ans->next;
        }
       ans->next=head1?head1:head2;
        return ans1->next;
    } 


int main() {
    int t;
    srand(time(0));
    cin>>t;
    ListNode* tmp,*head;
    tmp = new ListNode(-1);
    head = tmp;
    for (int i=0;i<t;i++) {
        int data = rand()%100;
//      int data;
//      cin>>data;
        cout<<data<<" ";
        ListNode* k = new ListNode(data);
        tmp->next = k;
        tmp=tmp->next;
   }
   cout<<endl;
   head = sortList(head->next);
   while(head) {
    cout<<head->val<<" ";
    head = head->next;
   }
}