題意

題解1

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

題解2：遞歸

def number_to_string(num):
    # Return a string of the number here!
    if num < 0:  # 處理負(fù)數(shù)
        return "-" + number_to_string(-num)
    if num < 10:  # 只有1位，直接處理
        return chr(ord('0') + num)
    num, r = divmod(num, 10)  # 高位數(shù)及個(gè)位數(shù)
    return number_to_string(num) + number_to_string(r)