LeetCode -- Integer to Roman

Question:

Given an integer, convert it to a roman numeral.

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

Analysis：

给出一个整数，将它转换成罗马数字。

注意：保证输入的数字在1~3999之间。

思路： 主要是首先要知道罗马数字与阿拉伯数字之间的转换关系。具体规则自行百度。观察规律可知，只有4，9，40，90……是由右面大的减去左边小的，其他的都是在右边直接累加。因此将所有特殊情况罗列出来即可。

1. 递归的解决方案。

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

2. 非递归的解决方案。

public class Solution {
    public String intToRoman(int num) {
        String[] str = {"I", "IV", "V", "IX", "X", "XL", "L", "XC", "C", "CD", "D", "CM", "M"};
        int[] val = {1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000};
        String result = "";
        for(int i=val.length-1; num != 0; i--) {
            while(num >= val[i]) {
                num -= val[i];
                result += str[i];
            }
        }
        return result;
    }
}