C++程序运行出现错误，在devc++提示permission denied。在vs2015提示debug assertion failed，求解决！！

#include <iostream>
#include <string>
using namespace std;
int main()
{
int j, i;
string a, b;
cout << "please enter two chars that you want to connact:";
cin >> a >> b;
for (i = 0; a[i] != '\n'; i++) i++;
for (j = 0; b[j] != '\n'; )
a[i++] = b[j++];
cout << "the new char is :" << a << endl;
return 0;
}