main.go

// Source: https://leetcode.com/problems/roman-to-integer
// Title: Roman to Integer
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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.
// ```
//
// **Example 2:**
//
// ```
// Input: s = "LVIII"
// Output: 58
// Explanation: L = 50, V= 5, III = 3.
// ```
//
// **Example 3:**
//
// ```
// Input: s = "MCMXCIV"
// Output: 1994
// Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
// ```
//
// **Constraints:**
//
// - `1 <= s.length <= 15`
// - `s` contains onlythe characters `('I', 'V', 'X', 'L', 'C', 'D', 'M')`.
// - It is **guaranteed**that `s` is a valid roman numeral in the range `[1, 3999]`.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

func romanToInt(s string) int {
	romans := map[byte]int{
		'I': 1,
		'V': 5,
		'X': 10,
		'L': 50,
		'C': 100,
		'D': 500,
		'M': 1000,
	}

	ans := 0
	prev := 0

	n := len(s)
	for i := n - 1; i >= 0; i-- {
		curr := romans[s[i]]
		if curr >= prev {
			ans += curr
		} else {
			ans -= curr
		}
		prev = curr
	}

	return ans
}