题意

题解1

def number_to_string(num):
    # Return a string of the number here!
    return str(num)

题解2：递归

def number_to_string(num):
    # Return a string of the number here!
    if num < 0:  # 处理负数
        return "-" + number_to_string(-num)
    if num < 10:  # 只有1位，直接处理
        return chr(ord('0') + num)
    num, r = divmod(num, 10)  # 高位数及个位数
    return number_to_string(num) + number_to_string(r)