12. Integer to Roman

Published on February 16, 2024

Question

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, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 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.

Example 1:

Input: num = 3
Output: "III"
Explanation: 3 is represented as 3 ones.

Example 2:

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

Example 3:

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

Constraints:

1 <= num <= 3999

Approach

Same idea like question 13, set a map to store the info;
Start from the largest number, do the reduction

Code

class Solution {
    private static final int[] values = {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};    
    private static final String[] symbols = {"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};

    public String intToRoman(int num) {
        StringBuilder sb = new StringBuilder();
        // Loop through each symbol, stopping if num becomes 0.
        for (int i = 0; i < values.length && num > 0; i++) {
            // Repeat while the current symbol still fits into num.
            while (values[i] <= num) {
                num -= values[i];
                sb.append(symbols[i]);
            }
        }
        return sb.toString();
    }
}

Code

class Solution {
    public String intToRoman(int num) {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "I");
        map.put(4, "IV");
        map.put(5, "V");
        map.put(9, "IX");
        map.put(10, "X");
        map.put(40, "XL");
        map.put(50, "L");
        map.put(90, "XC");
        map.put(100, "C");
        map.put(400, "CD");
        map.put(500, "D");
        map.put(900, "CM");
        map.put(1000, "M");
        
        StringBuilder sb = new StringBuilder();
        while (num > 0) {
            if (num >= 1000) {
                num -= 1000;
                sb.append(map.get(1000));
            } else if (num >= 900) {
                num -= 900;
                sb.append(map.get(900));
            } else if (num >= 500) {
                num -= 500;
                sb.append(map.get(500));
            } else if (num >= 400) {
                num -= 400;
                sb.append(map.get(400));
            } else if (num >= 100) {
                num -= 100;
                sb.append(map.get(100));
            } else if (num >= 90) {
                num -= 90;
                sb.append(map.get(90));
            } else if (num >= 50) {
                num -= 50;
                sb.append(map.get(50));
            } else if (num >= 40) {
                num -= 40;
                sb.append(map.get(40));
            } else if (num >= 10) {
                num -= 10;
                sb.append(map.get(10));
            } else if (num >= 9) {
                num -= 9;
                sb.append(map.get(9));
            } else if (num >= 5) {
                num -= 5;
                sb.append(map.get(5));
            } else if (num >= 4) {
                num -= 4;
                sb.append(map.get(4));
            } else if (num >= 1) {
                num -= 1;
                sb.append(map.get(1));
            }
        }
        return sb.toString();
    }
}