[LeetCode] 12. Integer to Roman

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.

整数转换罗马数字。

13题是正好相反。题意是给一个整数，请把它转换成罗马数字。

这个题不涉及什么算法或者思想，只是单纯的转换。我给出一个我个人觉得比较好记的做法。先给出每个罗马数字和他们对应的整数之间的mapping，然后对于要处理的整数X，找到最大的小于X的对应的罗马数字。因为是JavaScript，所以这里可以直接拼接字符串而不用像Java那样先创建StringBuilder（12行）。

时间O(n)

空间O(n)

JavaScript实现

/**
 * @param {number} num
 * @return {string}
 */
var intToRoman = function (num) {
    var list = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
    var valueList = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
    var res = '';
    while (num !== 0) {
        for (var i = 0; i < valueList.length; i++) {
            if (num >= valueList[i]) {
                res += list[i];
                num -= valueList[i];
                break;
            }
        }
    }
    return res;
};

Java实现

class Solution {
    public String intToRoman(int num) {
        int[] values = {1000,900,500,400,100,90,50,40,10,9,5,4,1};
        String[] strs = {"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};

        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < values.length; i++) {
            while (num >= values[i]) {
                num -= values[i];
                sb.append(strs[i]);
            }
        }
        return sb.toString();
    }
}

LeetCode 题目总结