程序代码：

#include <iostream>
#include <string>
#include <vector>
using namespace std;
typedef struct{
    int count;
    string str;
}Data_Type;
typedef vector<Data_Type>::iterator it;
vector<Data_Type> SubStr(string Str)//求子串
{
    int i, j, k = 1;
    Data_Type Da;
    vector<Data_Type> vec;   
    for(i = 2; i <= Str.length(); i ++)//子串的长度
    {
        for(j = 0; j <= Str.length() - i; j ++)//从母串的下标开始
        {
            Da.str = Str.substr(j,i);
            Da.count = 1;
            vec.push_back(Da);
        }
    }
    return vec;
}
vector<Data_Type> Sub_Count(vector<Data_Type>&vec)
{
    vector<Data_Type> Sub;
    it vec_it1, vec_it2;
    it Sub_it1;
    for(vec_it1 = vec.begin(); vec_it1 != vec.end(); vec_it1 ++)
    {
        int flag;
        for(Sub_it1 = Sub.begin(); Sub_it1 != Sub.end(); Sub_it1 ++)
        {
            if(vec_it1->str == Sub_it1->str){flag = 0;break;}//当有相同的就马上退出
            else flag = 1;
        }
        if(flag)//如果在Sub容器中还没有该子串，那么就将它加到Sub容器中。
        {
            Sub.push_back(*vec_it1);//每扫描一个子串就将它加入另一个容器中，然后扫描到下一个子串的时候就跟Sub容器中才子串比较，如果相同，则说明该子串的个数已经计算过
            for(vec_it2 = vec_it1 + 1; vec_it2 != vec.end(); vec_it2 ++)   
                if( vec_it1->str == vec_it2->str) (Sub.end()-1)->count ++;                   
        }
    }
    return Sub;
}

int main()
{
   
    string Str;
    //int k = 1;
    vector<Data_Type> vec, Sub;
    it S_it;
    cout<<"请输入一个字符串:"<<endl;
    cin>>Str;
    vec = SubStr(Str);
    Sub = Sub_Count(vec);

    Data_Type max, Max;//用来保存出现次数最多的子串
    max.count = Sub.begin()->count;
    max.str = Sub.begin() ->str;
    for(S_it = Sub.begin(); S_it != Sub.end(); S_it ++)
    {
        if(S_it->count > max.count)
        {
            max.count = S_it->count;
            max.str = S_it ->str;
        }
    }
    Max.count = max.count;

    Max.str = max.str;

    for(S_it = Sub.begin(); S_it != Sub.end(); S_it ++)
    {
        if(S_it->count == Max.count && S_it->str.length() > Max.str.length())
        {
            Max.str = S_it->str;
            Max.count = S_it->count;       
        }
    }
    cout<<"子串个数最多且长度最大的是:"<<Max.str<<endl;
    return 0;
}