experience

f,如果n<3 f(n) = n;如果n>=3,那面f(n)= f(n-1) + 2f(n-2) + 3f(n-3);
----------------------------------------------------------
G(x){
if(x>=3) {
return G(x-1) + 2G(x-2) + 3G(x+3);
}
if(x<3){
return x;
}
}
-----------------------------------------------------------
G(n){
if(n < 3) return n;
return F(2, 1, 0, n);
}
F(a, b, c, n){
if(n < 3) return a;
return F(a + 2b + 3c, a, b, n-1);
}