remove-duplicates-from-sorted-list-ii

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.
For example,
Given1->2->3->3->4->4->5, return1->2->5.
Given1->1->1->2->3, return2->3.
本来是一道水题，结果彻底被教做人了。也反应了我的链表操作不太熟悉！
本题看起来虽然简单但是许多细节要注意！
首先是要记录头结点，其次是链表链接是要在不删除的情况下载连接，连接的时候，不要直接用p浅拷贝，因为p中的next值很容易造成错误，本来这条链是没有连起来的，结果连起来了。
注意结点的移动和结点的修改的区别。p->next = .., p = p->…。！

#include <iostream>using namespace std; struct ListNode {      int val;      ListNode *next;      ListNode(int x) : val(x), next(NULL) {}  };ListNode *deleteDuplicates(ListNode *head) {         ListNode *  pp  = new ListNode(-1); pp->next = head;         ListNode *  p   = head;         ListNode * ans=NULL;         int first = 1;            while (p) {                   ListNode * pt = p ;                   int flag = 0;                   while(pt->next&&pt->val==pt->next->val) {                       pt->next=pt->next->next;                        flag = 1;                   }                   if(!flag) {                              pp->next = new ListNode(p->val);                    pp=pp->next;                    p=p->next;                     if(first) {                        ans = pp;                        first = 0;                      }                   } else {                     p = p->next;                   }            }            return ans;} void printList(ListNode* head) {        ListNode* tmp = head;        while(tmp) {            cout<<tmp->val<<" ";            tmp = tmp->next;         }        cout<<endl;}int main() {    int t;    cin>>t;    while(t--) {        cout<<"请输入要输入链表的长度\n";        int n;        cin>>n;        int a; cin>>a;         ListNode* head = new ListNode(a);         ListNode* p = head;         for (int i=1;i<n;i++) {              cin>>a;             ListNode* tmp = new ListNode(a);            p->next = tmp;            p=p->next;        }         cout<<"处理前的链表为：\n";        printList(head);        cout<<"处理后的链表为：\n";        head = (*deleteDuplicates)(head);            printList(head);    }}