RomanToInteger.php

<?php

namespace App;

/**
 * Roman to Integer
 *
 * 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 ones 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 a roman numeral, convert it to an integer.
 *
 * Example 1:
 *
 * Input: s = "III"
 * Output: 3
 * Explanation: III = 3.
 *
 * https://leetcode.com/problems/roman-to-integer/
 */
class RomanToInteger
{
    private const ROMAN = [
        'I' => 1,
        'V' => 5,
        'X' => 10,
        'L' => 50,
        'C' => 100,
        'D' => 500,
        'M' => 1000
    ];

    /**
     * @param string $str
     * @return int
     */
    public function romanToInt(string $str): int
    {
        $strLength = strlen($str);
        $summa = 0;
        for ($i = 0; $i < $strLength; $i++) {
            if (($i + 1) < $strLength && self::ROMAN[$str[$i]] < self::ROMAN[$str[$i + 1]]) {
                $summa -= self::ROMAN[$str[$i]];
            } else {
                $summa += self::ROMAN[$str[$i]];
            }
        }
        return $summa;
    }
}