整数转为罗马数字

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

分析：

　　罗马数字不能表示负数和零，题目中已经给出输入数字为1~3999。罗马数字的表示规则可以上网查一下，需要考虑的是4和9两个数字，表示比较特殊。

class Solution(object):
    def intToRoman(self, num):
        """
        :type num: int
        :rtype: str
        """
        r = (1000,900,500,400,100,90,50,40,10,9,5,4,1)
        s = ('M','CM','D','CD','C','XC','L','XL','X','IX','V','IV','I')
        res = ''
        for i in range(len(r)):
            while num >= r[i] and num > 0:
                num = num - r[i]
                res += s[i]
        return res