判断一个长字符串中有几个长度为二的子串

#include
#include
using namespace std;
int fan(string &str,string &str1)
{
int ct = 0;

for (int i = 0, j = 1;str[j] != '\0';i++, j++)
{
	if (str[i] == str1[0] && str[j] == str1[1])
		ct++;
}
return ct;

}
int main()
{
string str;
string str2;
cin >> str;
cin.clear();
cin >> str2;
cout<<fan(str, str2);
return 0;
}