Roman to integer

Add each symbol's value, except when a smaller symbol sits just before a larger one: then subtract it instead.

The idea

Normally roman numerals are read left to right and added up. The one exception is a subtractive pair like IV or XC, where a smaller symbol just before a larger one means "subtract the smaller". Checking one symbol ahead catches every such pair in a single left-to-right pass.

Code

public static int RomanToInt(string s) {
    var val = new Dictionary<char, int> { ['I']=1,['V']=5,['X']=10,['L']=50,['C']=100,['D']=500,['M']=1000 };
    int total = 0;
    for (int i = 0; i < s.Length; i++) {
        int cur = val[s[i]];
        if (i + 1 < s.Length && cur < val[s[i + 1]])
            total -= cur;
        else
            total += cur;
    }
    return total;
}

Tips

• Only the current symbol needs to look ahead by one; nothing needs to look backward.
• MCMXCIV demonstrates all three subtractive pairs used in practice: CM, XC and IV.

In the .NET library

There is no built-in conversion in .NET. A lookup table plus this one-ahead check is the standard approach, and it naturally rejects nothing, so malformed input should be validated separately if needed.