Python之青蛙跳台阶

一直青蛙可以调1个台阶或者一次跳2个台阶，一共N个台阶，有多少种跳法？

#!/usr/bin/env python
#coding=utf-8

def qingwa(step):
    if step <= 0:
        raise Exception('error')
    if step == 1:
        return 1
    if step == 2:
        return 2
    return qingwa(step - 1) + qingwa(step - 2)
    
def qingwa1(step):
    if step <= 0:
        raise Exception('error')
    if step == 1:
        return 1
    if step == 2:
        return 2
 
    list1 = []
    list1.append(0)
    list1.append(1)
    list1.append(2)
    
    for i in range(3, step + 1):
        list1.append(list1[i - 1] + list1[i - 2])
    return list1[-1]
    
def qingwa2(step):
    if step <= 0:
        raise Exception('error')
    if step == 1:
        return 1
    if step == 2:
        return 2
        
    pre1 = 2 
    pre2 = 1
    value = 0
    
    for i in range(3, step + 1):
        value = pre1 + pre2
        pre2 = pre1
        pre1 = value
        
    return value
        
   


if __name__ == '__main__':
   
    print(qingwa2(9))
    print(qingwa(-1))