Another approach

string dupl(string x, unsigned n){ string r; // null by default if (n) { r = dupl(x, n/2); r += r; if (n % 2) r += x; } return r;}

string dupl(string x, unsigned n) { string r; // null by default if (n) { r = dupl(x, n/2); r += r; if (n % 2) r += x; } return r; }