Other:一只青蛙一次可以跳上1级台阶，也可以跳上2级……它也

public int JumpFloorII(int target) {
        if (target==0) {
            return 1;
        }
        if (target==1) {
            return 1;
        }
        return JumpFloorII(target-1)*2;
    }