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LeetCode problem: 12. Integer to Roman
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‎src/main/java/com/bl/integer_to_roman/README.md‎

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# 12. Integer to Roman
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Difficulty: `Medium`
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Topics: `Hash Table`, `Math`, `String`
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Roman numerals are represented by seven different symbols: `I`, `V`, `X`, `L`, `C`, `D` and `M`.
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```text
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Symbol Value
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I 1
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V 5
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X 10
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L 50
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C 100
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D 500
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M 1000
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```
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For example, `2` is written as `II` in Roman numeral, just two ones added together. `12` is written as `XII`, which is
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simply `X + II`. The number `27` is written as `XXVII`, which is `XX + V + II`.
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Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not `IIII`.
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Instead, the number four is written as `IV`. Because the one is before the five we subtract it making four. The same
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principle applies to the number nine, which is written as `IX`. There are six instances where subtraction is used:
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- `I` can be placed before `V` (5) and `X` (10) to make 4 and 9.
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- `X` can be placed before `L` (50) and `C` (100) to make 40 and 90.
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- `C` can be placed before `D` (500) and `M` (1000) to make 400 and 900.
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Given an integer, convert it to a roman numeral.
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**Example 1:**
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```text
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Input: num = 3
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Output: "III"
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Explanation: 3 is represented as 3 ones.
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```
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**Example 2:**
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```text
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Input: num = 58
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Output: "LVIII"
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Explanation: L = 50, V = 5, III = 3.
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```
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**Example 3:**
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```text
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Input: num = 1994
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Output: "MCMXCIV"
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Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
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```
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**Constraints:**
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- `1 <= num <= 3999`

‎src/main/java/com/bl/integer_to_roman/Solution.java‎

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package com.bl.integer_to_roman;
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/**
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* This is the solution to the LeetCode problem: 12. Integer to Roman
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*
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* @author Børre A. Opedal Lunde
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* @since 2024.01.27
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*/
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public class Solution {
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// This class stores the Roman numeral and its corresponding integer value.
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// (First I used a map, but the solution became really clumsy.)
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private static class RomanNumeral {
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private final int value;
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private final String numeral;
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public RomanNumeral(final int number, final String symbol) {
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this.value = number;
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this.numeral = symbol;
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}
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// No getters are needed because it's a private inner class.
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}
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// The Roman numerals are sorted in descending order because we want to
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// start with the largest and subtract our way down.
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private static final RomanNumeral[] ROMAN_NUMERALS = {
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new RomanNumeral(1000, "M"),
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new RomanNumeral(900, "CM"),
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new RomanNumeral(500, "D"),
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new RomanNumeral(400, "CD"),
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new RomanNumeral(100, "C"),
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new RomanNumeral(90, "XC"),
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new RomanNumeral(50, "L"),
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new RomanNumeral(40, "XL"),
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new RomanNumeral(10, "X"),
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new RomanNumeral(9, "IX"),
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new RomanNumeral(5, "V"),
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new RomanNumeral(4, "IV"),
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new RomanNumeral(1, "I")
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};
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public String intToRoman(int num) {
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// It is faster to use a string builder than a string.
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final StringBuilder builder = new StringBuilder();
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// Go through every Roman numeral and subtract our way down, while
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// appending the Roman numeral to the string builder.
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for (final RomanNumeral romanNumeral : ROMAN_NUMERALS) {
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while (num >= romanNumeral.value) {
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num -= romanNumeral.value;
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builder.append(romanNumeral.numeral);
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}
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}
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return builder.toString();
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}
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}

‎src/test/java/com/bl/integer_to_roman/SolutionTest.java‎

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package com.bl.integer_to_roman;
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import org.junit.jupiter.api.DisplayName;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.Arguments;
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import org.junit.jupiter.params.provider.MethodSource;
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import java.util.stream.Stream;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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/**
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* This is the test to the LeetCode problem: 12. Integer to Roman
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*
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* @author Børre A. Opedal Lunde
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* @since 2024.01.27
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*/
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@DisplayName("Integer to Roman")
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class SolutionTest {
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final Solution solution = new Solution();
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@Test
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@DisplayName("Example one")
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void exampleOne() {
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int num = 3;
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String expected = "III";
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String actual = solution.intToRoman(num);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Example two")
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void exampleTwo() {
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int num = 58;
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String expected = "LVIII";
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String actual = solution.intToRoman(num);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Example three")
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void exampleThree() {
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int num = 1994;
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String expected = "MCMXCIV";
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String actual = solution.intToRoman(num);
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assertEquals(expected, actual);
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}
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static Stream<Arguments> providePrincipalNumbers() {
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return Stream.of(
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Arguments.of(1, "I"),
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Arguments.of(5, "V"),
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Arguments.of(10, "X"),
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Arguments.of(50, "L"),
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Arguments.of(100, "C"),
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Arguments.of(500, "D"),
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Arguments.of(1000, "M")
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);
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}
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@ParameterizedTest(name = "Number {0} is Roman numeral {1}")
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@DisplayName("Principal Roman numbers")
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@MethodSource("providePrincipalNumbers")
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void principalRomanNumerals(int num, String expected) {
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String actual = solution.intToRoman(num);
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assertEquals(expected, actual);
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}
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static Stream<Arguments> provideSubtractiveNumbers() {
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return Stream.of(
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Arguments.of(4, "IV"),
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Arguments.of(9, "IX"),
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Arguments.of(40, "XL"),
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Arguments.of(90, "XC"),
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Arguments.of(400, "CD"),
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Arguments.of(900, "CM")
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);
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}
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@ParameterizedTest(name = "Number {0} is Roman numeral {1}")
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@DisplayName("Subtractive Roman numbers")
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@MethodSource("provideSubtractiveNumbers")
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void subtractiveRomanNumerals(int num, String expected) {
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String actual = solution.intToRoman(num);
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assertEquals(expected, actual);
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}
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static Stream<Arguments> provideNumbersFromOneToOneHundred() {
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return Stream.of(
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Arguments.of(1, "I"),
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Arguments.of(2, "II"),
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Arguments.of(3, "III"),
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Arguments.of(4, "IV"),
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Arguments.of(5, "V"),
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Arguments.of(6, "VI"),
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Arguments.of(7, "VII"),
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Arguments.of(8, "VIII"),
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Arguments.of(9, "IX"),
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Arguments.of(10, "X"),
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Arguments.of(11, "XI"),
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Arguments.of(12, "XII"),
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Arguments.of(13, "XIII"),
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Arguments.of(14, "XIV"),
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Arguments.of(15, "XV"),
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Arguments.of(16, "XVI"),
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Arguments.of(17, "XVII"),
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Arguments.of(18, "XVIII"),
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Arguments.of(19, "XIX"),
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Arguments.of(20, "XX"),
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Arguments.of(21, "XXI"),
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Arguments.of(22, "XXII"),
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Arguments.of(23, "XXIII"),
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Arguments.of(24, "XXIV"),
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Arguments.of(25, "XXV"),
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Arguments.of(26, "XXVI"),
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Arguments.of(27, "XXVII"),
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Arguments.of(28, "XXVIII"),
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Arguments.of(29, "XXIX"),
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Arguments.of(30, "XXX"),
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Arguments.of(31, "XXXI"),
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Arguments.of(32, "XXXII"),
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Arguments.of(33, "XXXIII"),
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Arguments.of(34, "XXXIV"),
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Arguments.of(35, "XXXV"),
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Arguments.of(36, "XXXVI"),
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Arguments.of(37, "XXXVII"),
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Arguments.of(38, "XXXVIII"),
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Arguments.of(39, "XXXIX"),
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Arguments.of(40, "XL"),
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Arguments.of(41, "XLI"),
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Arguments.of(42, "XLII"),
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Arguments.of(43, "XLIII"),
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Arguments.of(44, "XLIV"),
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Arguments.of(45, "XLV"),
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Arguments.of(46, "XLVI"),
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Arguments.of(47, "XLVII"),
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Arguments.of(48, "XLVIII"),
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Arguments.of(49, "XLIX"),
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Arguments.of(50, "L"),
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Arguments.of(51, "LI"),
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Arguments.of(52, "LII"),
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Arguments.of(53, "LIII"),
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Arguments.of(54, "LIV"),
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Arguments.of(55, "LV"),
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Arguments.of(56, "LVI"),
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Arguments.of(57, "LVII"),
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Arguments.of(58, "LVIII"),
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Arguments.of(59, "LIX"),
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Arguments.of(60, "LX"),
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Arguments.of(61, "LXI"),
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Arguments.of(62, "LXII"),
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Arguments.of(63, "LXIII"),
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Arguments.of(64, "LXIV"),
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Arguments.of(65, "LXV"),
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Arguments.of(66, "LXVI"),
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Arguments.of(67, "LXVII"),
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Arguments.of(68, "LXVIII"),
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Arguments.of(69, "LXIX"),
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Arguments.of(70, "LXX"),
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Arguments.of(71, "LXXI"),
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Arguments.of(72, "LXXII"),
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Arguments.of(73, "LXXIII"),
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Arguments.of(74, "LXXIV"),
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Arguments.of(75, "LXXV"),
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Arguments.of(76, "LXXVI"),
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Arguments.of(77, "LXXVII"),
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Arguments.of(78, "LXXVIII"),
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Arguments.of(79, "LXXIX"),
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Arguments.of(80, "LXXX"),
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Arguments.of(81, "LXXXI"),
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Arguments.of(82, "LXXXII"),
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Arguments.of(83, "LXXXIII"),
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Arguments.of(84, "LXXXIV"),
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Arguments.of(85, "LXXXV"),
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Arguments.of(86, "LXXXVI"),
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Arguments.of(87, "LXXXVII"),
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Arguments.of(88, "LXXXVIII"),
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Arguments.of(89, "LXXXIX"),
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Arguments.of(90, "XC"),
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Arguments.of(91, "XCI"),
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Arguments.of(92, "XCII"),
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Arguments.of(93, "XCIII"),
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Arguments.of(94, "XCIV"),
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Arguments.of(95, "XCV"),
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Arguments.of(96, "XCVI"),
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Arguments.of(97, "XCVII"),
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Arguments.of(98, "XCVIII"),
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Arguments.of(99, "XCIX"),
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Arguments.of(100, "C")
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);
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}
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@ParameterizedTest(name = "Number {0} is Roman numeral {1}")
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@DisplayName("Roman numerals from one to one hundred")
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@MethodSource("provideNumbersFromOneToOneHundred")
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void romanNumeralsFromOneToOneHundred(int num, String expected) {
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String actual = solution.intToRoman(num);
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assertEquals(actual, expected);
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}
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}

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