LC13. Roman to Integer

import (
    "fmt"
)
 
func romanToInt(s string) int {
    total := 0
 
    for i := 0; i < len(s); {
        dec, size := decodeRomanNumeral(s[i:])
        total += dec
        i += size
    }
 
    return total
}
 
var m map[string]int = map[string]int {
    "I": 1,
    "IV": 4,
    "V": 5,
    "IX": 9,
    "X": 10,
    "XL": 40,
    "L": 50,
    "XC": 90,
    "C": 100,
    "CD": 400,
    "D": 500,
    "CM": 900,
    "M": 1000,
}
 
func decodeRomanNumeral(s string) (int, int) {
    oneLetter := string(s[0])
    twoLetter := ""
 
    if len(s) > 1 {
        twoLetter = s[:2]
    }
 
    if dec, ok := m[twoLetter]; ok {
        return dec, 2
    }
 
    if dec, ok := m[oneLetter]; ok {
        return dec, 1
    }
 
    panic(fmt.Sprintf("invalid character in %s", s))
}

Complexity

Time: $O(n)$
Space: $O(1)$