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Interview Question Roman to Integer

Last update: 12/27/2016 4:23:00 AM


The problem is to convert a roman numeral to an integer value.

Problem #13 - Roman to Integer

Description: Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Difficulty: Easy

Solution in Java

Reference Roman Table


It is useful to remember the following simple rule before starting to solve this problem. A roman numeral can be calculated by adding or subtracting the numeric values of the characters each other by comparing their values. The rule is basically if the value of the character is greater than the value of the next character, the total sum variable is added with the value of the character. Otherwise the subtraction operation is performed.

For example,

So, we can implement this rule as below.

package com.fatihkabakci.Easy.RomanToInteger;

 * @author fkabakci
 * Problem Description: Given a roman numeral, convert it to an integer.
 * Input is guaranteed to be within the range from 1 to 3999.
 * Solution:
 * Postcondition: num is within the range from 1 to 3999
 * 1. Compare adjacent two characters in regarding to the following rule.
 * 2. If the roman value of a character is greater than the value of another character which is right of that,
 *    	Total sum is added with the value of the first character.
 *    otherwise,
 *    	Total sum is subtracted by the value of the first character.
 * 3. Go back to 2 until all characters are visited.
 * Example: 
 * Roman characters: MCMXCVI
 * Their values: 1000 100 1000 10 100 5 1
 * 1. Start comparing with 1000 and 100, because of the comparison 1000 > 100 is true, add total sum with 1000, total = 1000
 * 2. Then 100 and 1000. 100 < 1000, subtract total sum by 100, total = 900
 * 3. Now is 1000 and 10. 1000 > 10, adding total sum with 1000, total = 1900
 * 4. 10 and 100. 10 < 100, again subtracting total sum by 10, total = 1890
 * 5. 100 and 5. 100 > 5, adding total sum with 100, total = 1990
 * 6. 5 and 1. 5 > 1, adding total sum with 5, total = 1995,
 * 7. Lastly, 1 and 0 because there is no more character left. 1 > 0, adding sum with 1 more, total = 1996.
public class RomanToInteger {	
	private int getValue(char c) {
		switch (c) {
		case "M":
			return 1000;
		case "D":
			return 500;
		case "C":
			return 100;
		case "L":
			return 50;
		case "X":
			return 10;
		case "V":
			return 5;
		case "I":
			return 1;
			return 0;

	public int romanToInt(String s) {
		int acc = 0, i = 0, l = s.length();
		while (i < l) {
			char c = s.charAt(i);
			char csub = (i + 1 < s.length()) ? s.charAt(i + 1) : "\0";
			int v1 = getValue(c);
			int v2 = getValue(csub);
			if (v1 < v2)
				acc = acc - v1;
				acc = acc + v1;
			i = i + 1;
		return acc;

	public static void main(String[] args) {
		RomanToInteger rti = new RomanToInteger();

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#12 Integer To Roman

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