递归算法过程

function f(num) {
            if (num <= 1) {
                return 1;
            } else {
                return 2 * f(num - 1);
            }
        }
        alert(f(6));

运算过程：

f(6)  = f(5) * 2 = (2*32)

f(5)  = f(4) * 2 = (2*16)

f(4)  = f(3) * 2 = (2*8)

f(3)  = f(2) * 2 = (2*4)

f(2)  = f(1) * 2 = (2*1)

f(1)  = 1