LeetCode_Medium_12. Integer to Roman

2019.1.26

题目描述：

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.

这题与之前的罗马数字转整数是对应的，转换规则具体参照博客：https://blog.csdn.net/weixin_41637618/article/details/86373369，就不细说了。

解法一：

因为题目限制了整数范围是1-3999，所以我们可以将1-1000对应的罗马字符都枚举出来，利用贪婪算法的思想，每次找出整数中的最大值输出并减去其值，再循环下去。

C++代码：

class Solution {
public:
    string intToRoman(int num) {
        int values[]={1000,900,500,400,100,90,50,40,10,9,5,4,1};
        string reps[]={"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};
        string res;
        for(int i=0; i<13; i++){
            while(num>=values[i]){
                num -= values[i];
                res += reps[i];
            }
        }
        return res;
    }
};

解法二：

提交完看讨论，有个很有意思的解法，是将所有的情况全部都枚举出来，然后按位查找。

C++代码：

class Solution {
public:
    string intToRoman(int num) {
        string res = "";
        vector<string> v1{"", "M", "MM", "MMM"};
        vector<string> v2{"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
        vector<string> v3{"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
        vector<string> v4{"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
        return v1[num / 1000] + v2[(num % 1000) / 100] + v3[(num % 100) / 10] + v4[num % 10];
    }
};

LeetCode_Medium_12. Integer to Roman

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