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代码说明：

说明：  一只青蛙一次可以跳上1级台阶，也可以跳上2级。求该青蛙跳上一个n级的台阶总共有多少种跳法。要求：分别用普通方法和递归方法实现(A frog can jump up a step or two at a time. Find out how many jumping methods the frog can use to jump up an n-step. Requirements: Implemented by common method and recursive method respectively)