Contents.swift

/// 13. Integer to Roman
/// 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, two is written as II in Roman numeral, just two one's added together. Twelve is
/// written as, XII, which is simply X + II. The number twenty seven 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. Input is guaranteed to be within the range from
/// 1 to 3999.

import XCTest

func romanToInt(_ s: String) -> Int {
    let dict = [
        "M": 1000, "CM": 900, "D": 500, "CD": 400, "C": 100, "XC": 90,
        "L": 50, "XL": 40, "X": 10, "IX": 9, "V": 5, "IV": 4, "I": 1,
        ]
    var roman = Array(s)
    var result = 0
    var p = 0
    while p < roman.count {
        if p < roman.count - 1,
           let value = dict[String(roman[p]) + String(roman[p + 1])] {
            result += value
            p += 2
        } else if let value = dict[String(roman[p])] {
            result += value
            p += 1
        } else {
            fatalError()
        }
    }
    return result
}

class Tests: XCTestCase {
    func testExample() {
        XCTAssertEqual(romanToInt("III"), 3)
    }
}

Tests.defaultTestSuite.run()