飛飛（八十）（补）请构造String类的加、减运算(s1+s2将两个字符串的连接起来；s1-s2是将s1的尾部空格和s2的前导空格去除后的连接)

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/**烟台大学计算机学院*文件名称：*作    者：李飞*完成日期：2016年5月23日*版 本 号：codeblocks 16.01**问题描述：*程序输入：无*程序输出：*/#include <iostream>#include<Cstring>using namespace std;class String{public:    String();//默认构造函数    String(char *s);    String(String &str);//构造函数    ~String();    void display();    friend String operator+(String &s1,String &s2);    friend String operator-(String &s1,String &s2);private:    char *p;   //指向存储的字符串    int len;   //记录字符串的长度};String::String(){    len=0;    p=NULL;}String::String(char *s){    len=strlen(s);    p=new char[len+1];   strcpy(p,s);}String::String(String &str){    len=str.len;    if(p!=NULL) delete []p;    p=new char[len+1];    strcpy(p,str.p);}String::~String(){    if(!p)delete []p;}void String::display( ){    cout<<p<<endl;}String operator+(String &s1,String &s2){    String s;    s.len=s1.len+s2.len;    s.p=new char[s.len+1];    strcpy(s.p,s1.p);    strcat(s.p,s2.p);    return s;}String operator - (String &s1, String &s2 ){    String s;    char *c1=new char[s1.len+1];    strcpy(c1,s1.p);    int i=s1.len-1;    while(i>=0&&c1[i]==' ') --i;    c1[i+1]='\0';  char *c2=new char[s2.len+1];    strcpy(c2,s2.p);    i=0;    while(i<s2.len&&c2[i]==' ') ++i;   int j=0;    while(i<s2.len&&c2[i]!='\0')    {        c2[j]=c2[i];        ++i;    }    c2[j]='\0';    //将这两部分接起来    s.len = strlen(c1)+strlen(c2);    s.p = new char[s.len+1];    strcpy(s.p,c1);    strcat(s.p,c2);    delete c1;    delete c2;    return s;}int main(){    String string1("Hello "),string2(" World");    string1.display();    string2.display();    String string3;    string3=string1+string2;    string3.display();    string3=string1-string2;    string3.display();    return 0;}

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