Python:将这个递归函数转换为迭代。

def g(n):
"""Return the value of G(n), computed recursively.

>>> g(1)
1
>>> g(2)
2
>>> g(3)
3
>>> g(4)
10
>>> g(5)
22
"""
if n<=3:
    return n
else:
    return g(n-1)+2*g(n-2)+3*g(n-3)
def g(n):
"""Return the value of G(n), computed