题目

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

• I can be placed before V (5) and X (10) to make 4 and 9. 
• X can be placed before L (50) and C (100) to make 40 and 90. 
• C can be placed before D (500) and M (1000) to make 400 and 900.

Given an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: 3
Output: "III"

Example 2:

Input: 4
Output: "IV"

Example 3:

Input: 9
Output: "IX"

Example 4:

Input: 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.

Example 5:

Input:1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

 解题思路

题目不难，而且也是大家在生活中常见的阿拉伯数字到罗马数字转换。我的解法就是借助于初始定义一个数组，用于存放所有特殊转换的阿拉伯数字，加一个map集合，用于存放这些特殊的阿拉伯数字到罗马数字的映射关系。

之后便是对数组从大到小的遍历，一旦给出的数字大于当前数组中的数字，便在返回值中对应追加上对应罗马数字，并且把给定数字减去当前数组中的值，遍历结束后，所得字符串即为整型到罗马数字的转换结果。

代码实现

    int[] nums = {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
    Map<Integer, String> map = new HashMap<>();

    public String intToRoman(int num) {
        map.put(1, "I");
        map.put(5, "V");
        map.put(10, "X");
        map.put(50, "L");
        map.put(100, "C");
        map.put(500, "D");
        map.put(1000, "M");
        map.put(4, "IV");
        map.put(9, "IX");
        map.put(40, "XL");
        map.put(90, "XC");
        map.put(400, "CD");
        map.put(900, "CM");

        String s = "";
        for (int i = 0; i < nums.length; i++) {
            while (num / nums[i] > 0) {
                s += map.get(nums[i]);
                num -= nums[i];
            }
        }
        return s;
    }