C + + polymorphism get data merge from different data sources

Define a base class, with pointers to the base class pointing to different subclasses ...

#include <iostream> #include <stdio.h> #include <algorithm> #include <vector>using namespace  Std;class ListNode {public:int val;  Listnode* Next;  ListNode (int x): Val (x), Next (NULL) {}};class DataSource {public:listnode* cur; DataSource (): cur (NULL) {} virtual listnode* getData () = 0;};     Class Disk:public DataSource {public:listnode* getData () {//maybe other implements cur = NULL;   return NULL;     }};class mem:public DataSource {public:listnode* getData () {//maybe other implements cur = NULL;   return NULL; }};class cmp {Public:bool operator () (datasource* A, datasource* b) {return a->cur->val > B->cur->val  ; }};class Solution {Public:listnode *mergeklists (vector<datasource *> &lists) {//note:the Solution obj    ECT is instantiated only once and are reused by each test case.    ListNode *head = null, *cur = NULL;    Vector<datasource*>::iterator it = Lists.begin (); while (it ! = Lists.end ()) {if ((*it)->getdata () = = NULL) it = lists.erase (it);    else it++;    } if (Lists.size () < 1) return NULL;    Make_heap (Lists.begin (), Lists.end (), CMP ());      while (Lists.size () > 0) {if (head = = NULL) head = cur = lists[0]->cur;        else {Cur->next = lists[0]->cur;      Cur = cur->next;                 } pop_heap (Lists.begin (), Lists.end (), CMP ());      if (Lists[lists.size ()-1]->getdata () = NULL) Push_heap (Lists.begin (), Lists.end (), CMP ());          else Lists.pop_back ();  } return head;  }};int Main () {datasource* D1 = new Disk ();  datasource* D2 = new Mem ();  Vector<datasource*> lists;  Lists.push_back (D1);  Lists.push_back (D2);  Solution S;  listnode* res = s.mergeklists (lists); return 0;}