递归算法问题：有一对兔子，从出生后第3个月起每个月都生一对兔子，小兔子长到第3个月每个月又生一对兔子，假如兔子都不死，问n（接收用户输入数据）个月后的兔子总数为多少？-Recursive algorithm problem: a pair of rabbits, from the first three months after birth month, a pair of rabbits are born, grow a small rabbit to the first three mont