string s1 = “hello, ”, s2 = “world\n”;
string s3 = s1 + s2; // s3的内容是 hello, world\n
#include <iostream>
using namespace std;
int main()
{
string a, b;
getline(cin,a);
for(int i = 0; i < a.size(); i++)
b += a[i] + a[(i + 1) % a.size()];
cout << b << endl;
}