IntegerToRoman.java

package by.andd3dfx.numeric;

import java.util.LinkedHashMap;

/**
 * <pre>
 * <a href="https://leetcode.com/problems/integer-to-roman/description/">Task description</a>
 *
 * 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
 * </pre>
 *
 * @see <a href="https://youtu.be/83hLrZyfxeU">Video solution</a>
 */
public class IntegerToRoman {

    private static final LinkedHashMap<Integer, String> vocabulary = new LinkedHashMap<>() {{
        put(1000, "M");
        put(900, "CM");
        put(500, "D");
        put(400, "CD");
        put(100, "C");
        put(90, "XC");
        put(50, "L");
        put(40, "XL");
        put(10, "X");
        put(9, "IX");
        put(5, "V");
        put(4, "IV");
        put(1, "I");
    }};

    public static String intToRoman(int number) {
        if (number < 1 || number > 3999) {
            throw new IllegalArgumentException("Parameter `number` should belong interval: [1, 3999]");
        }

        var sb = new StringBuilder();
        for (var entry : vocabulary.entrySet()) {
            var key = entry.getKey();
            var value = entry.getValue();

            while (number >= key) {
                sb.append(value);
                number -= key;
            }
        }
        return sb.toString();
    }
}