C++程序设计题目，达人帮忙，谢谢

class string{
char* pStr;
public:
string();
~string();
string(const char* pString);
string(const string& rhs);
string(const string& rhs,unsigned int pos,unsigned int n);
string append(const char *s);
int compare(const string & str)const;
};
string::string(){
pStr=NULL;
}
string::~string(){
if(pStr!=NULL)
delete[] pStr;
}
string::string(const char* pString){
pStr=new char[strlen(pString)+1];
strcpy(pStr,pString);
}
string::string(const string& rhs){
pStr=new char[strlen(rhs.pStr)+1];
strcpy(pStr,rhs.pStr);
}
string::string(const string & rhs,unsigned int pos,unsigned int n){
pStr=new char[n+1];
strncpy(pStr,rhs.pStr+pos,n);
}
string string::append(const char* s){
char* temp=new char[strlen(pStr)+strlen(s)+1];
strcpy(temp,pStr);
strcat(temp,s);
delete[] pStr;
pStr=temp;
return *this;<