Integer to Roman


#1

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.

#2

The below algorithm uses a pre-defined list of all integers to roman values mapping to generate a roman number for a given number.

Apart from having the basic numbers like 1000, 500, 100 etc, we should also have the numbers like 900, 400, 90 etc in our pre-defined list to have a short roman number.

For example, if you don’t have the numbers mentioned above, 9 will be VIIII instead of IX.

func intToRoman(num int) string {
    vals := []int{1,4, 5, 9 , 10, 40, 50, 90, 100, 400, 500, 900, 1000}
    m := map[int]string{
        1 : "I", 4 :"IV", 5 : "V", 9 : "IX", 10: "X",40 :"XL", 50: "L",90: "XC", 100: "C",400 : "CD", 500: "D", 900: "CM", 1000: "M",
    }
    
    var result strings.Builder
    for i:=len(vals)-1;i>=0;i--{
        for vals[i]<=num && num>0{
            num = num-vals[i]
            result.WriteString(m[vals[i]])
        }
    }

    return result.String()
}