编写从int到Romans的转换器是一个标准的面试问题。我曾经写过以下双向实现（toString--十进制到罗马; parse--罗马到十进制）。该实现满足了罗马数表示的一些额外标准，这些标准不是强制性的，但通常遵循：

'''
Created on Feb 7, 2013

@author: olegs
'''

ROMAN_CONSTANTS = (
            ( "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" ),
            ( "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" ),
            ( "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" ),
            ( "", "M", "MM", "MMM", "",   "",  "-",  "",    "",     ""   ),
        )

ROMAN_SYMBOL_MAP = dict(I=1, V=5, X=10, L=50, C=100, D=500, M=1000)

CUTOFF = 4000
BIG_DEC = 2900
BIG_ROMAN = "MMCM"
ROMAN_NOUGHT = "nulla"

def digits(num):
    if num < 0:
        raise Exception('range error: negative numbers not supported')
    if num % 1 != 0.0:
        raise Exception('floating point numbers not supported')
    res = []
    while num > 0:
        res.append(num % 10)
        num //= 10
    return res

def toString(num, emptyZero=False):
    if num < CUTOFF:
        digitlist = digits(num)
        if digitlist:
            res = reversed([ ROMAN_CONSTANTS[order][digit] for order, digit in enumerate(digitlist) ])
            return "".join(res)
        else:
            return "" if emptyZero else ROMAN_NOUGHT 
    else:
        if num % 1 != 0.0:
            raise Exception('floating point numbers not supported')
        # For numbers over or equal the CUTOFF, the remainder of division by 2900
        # is represented as above, prepended with the multiples of MMCM (2900 in Roman),
        # which guarantees no more than 3 repetitive Ms.
        return BIG_ROMAN * (num // BIG_DEC) + toString(num % BIG_DEC, emptyZero=True)

def parse(numeral):
    numeral = numeral.upper()
    result = 0
    if numeral == ROMAN_NOUGHT.upper():
        return result
    lastVal = 0
    lastCount = 0
    subtraction = False
    for symbol in numeral[::-1]:
        value = ROMAN_SYMBOL_MAP.get(symbol)
        if not value:
            raise Exception('incorrect symbol')
        if lastVal == 0:
            lastCount = 1
            lastVal = value
        elif lastVal == value:
            lastCount += 1
            # exceptions
        else:
            result += (-1 if subtraction else 1) * lastVal * lastCount
            subtraction = lastVal > value
            lastCount = 1
            lastVal = value
    return result + (-1 if subtraction else 1) * lastVal * lastCount

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def checkIfRomanNumeral(numeral):
"""Controls that the userinput only contains valid roman numerals"""
    numeral = numeral.upper()
    validRomanNumerals = ["M", "D", "C", "L", "X", "V", "I", "(", ")"]
    valid = True
    for letters in numeral:
        if letters not in validRomanNumerals:
            print("Sorry that is not a valid roman numeral")
            valid = False
            break
    return valid

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返回一个布尔值，无论给定的“numeric "是否是罗马数字。

除了已经指出的设计问题，我想回答这个问题为什么你的for循环没有遍历所有的数字
如果你的代码认为这些条目是有效的，那么循环进入elif子句，在那里它调用romanToInt(numeral)，然后调用break。
说明：只要满足本例中的条件，循环将停止通过i in list

for i in list:
   # do something
   if condition:
       break # "Stop the innermost loop now!"

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代替循环，你可以将输入和有效的文字都转换成集合，然后对它们进行子操作：

def checkIfRomanNumeral(numeral):
    numeral = {c for c in numeral.upper()}
    validRomanNumerals = {c for c in "MDCLXVI()"}
    return not numeral - validRomanNumerals

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如果numeral有效，则返回True，否则返回False。（假设空字符串有效）

有一种方法我建立的解决方案，你可以很容易地修改它，以您的需要，甚至得到的数字。我的程序是打印，如果从用户的字符串是一个有效的数字，但你可以很容易地改变它，以获得实际的值。

def main():
    # YOUR CODE GOES HERE
    # Please take input and print output to standard input/output (stdin/stdout)
    # E.g. 'input()/raw_input()' for input & 'print' for output
    N = 1
    for i in range(N):
        romanStr = input() # your str
        units = ["I", "II", "III" , 'IV' , 'V', 'VI', 'VII', 'VIII', "IX"]
        tens = ['X', "XX", "XXX", 'XL', 'L', 'LX', 'LXX', 'LXXX', 'XC']
        hundreds = ['C', 'CC', 'CCC', 'CD', 'D', 'DC', 'DCC', 'DCCC', 'CM']
        thousands = ['M', 'MM', 'MMM']
        allTypes = [units, tens, hundreds, thousands][::-1]
        isError = False
        lenRom = len(romanStr)
        copy = romanStr
        while len(copy) > 0 and len(allTypes) > 0:
            currentUnit = allTypes[0]
            # check units
            firstNum = copy[0]
            lastTwo = copy[:2]
            last3 = copy[:3]
            last4 = copy[:4]
            isLastTwo = lastTwo in currentUnit and len(lastTwo) == 2
            isLast3 = last3 in currentUnit and len(last3) == 3
            isLast4 = last4 in currentUnit and len(last4) == 4
            
            if (firstNum in currentUnit and not (isLastTwo or isLast3 or isLast4) ):
                copy = copy[1::]
                isError = False
            elif (isLastTwo and not (isLast3 or isLast4) ):
                copy = copy[2::]
                isError = False
            elif (isLast3 and not (isLast4) ):
                copy = copy[3::]
                isError = False
            elif (isLast4):
                copy = copy[4::]
                isError = False
            else:
                
                isError = True
                # copy = ''
            allTypes.pop(0)
        
        if (isError or len(copy) != 0 ):
            print("NO")
            return 0
        else:
            print("YES")
            return 1

    return 0

if __name__ == '__main__':
    main()

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我知道最初的海报是用Python写的，但我已经从上面引用了Oleg的答案，将其移植到PHP，并纠正了它的缺陷。
这段PHP代码应该正确地将罗马数字转换为数字，并且不应该允许转换无效的罗马数字。

<?php
define('ROMAN_VERBOSE', true);

class RomanNumeral {

protected static $lookup2 = [
   'I' => 1,
   'V' => 5,
   'X' => 10,
   'L' => 50,
   'C' => 100,
   'D' => 500,
   'M' => 1000,
];

public static function toNumber($numeral) {
if (!$numeral || !is_string($numeral))
  throw new \Exception("Invalid Roman numeral.");

$num = strtoupper(trim($numeral));
$nbr = 0;

if (ROMAN_VERBOSE === true)
  echo $num.PHP_EOL;

$lastVal = 0;
$lastCount = 0;
$lastGlyph = '';
$sub = false;

for ($x = 0; $x < strlen($num); $x++) {
  $val = self::$lookup2[$num[$x]] ?? 0;

  if (ROMAN_VERBOSE === true) {
    echo "********* ENTERING x = ".$x." *********".PHP_EOL;
    echo "Value: ".$val.PHP_EOL;
    echo "Last Count: ".$lastCount.PHP_EOL;
    echo "Last Value: ".$lastVal.PHP_EOL;
    echo "Glyph: ".$num[$x].PHP_EOL;
    echo "Last Glyph: ".$lastGlyph.PHP_EOL;
    echo "Current Number: ".$nbr.PHP_EOL;
    echo "Subtract: ".($sub !== false ? "Y" : "N").PHP_EOL;
    echo PHP_EOL;
  }

  if ($val == 0)
    throw new \Exception("Invalid Roman numeral.");
  if ($lastVal == 0) {
    $lastCount = 1;
    $lastVal = $val;
    $lastGlyph = $num[$x];
  } else if ($lastVal == $val) {
    $lastCount += 1;
    $sub = false;
  } else {
    if ($num[$x] == 'V' || $num[$x] == 'L' || $num[$x] == 'D')
      $sub = false;
    $nbr += ($sub === true ? -1 : 1) * ($lastVal * $lastCount);
    $sub = ($lastVal > $val);
    if ($num[$x] == 'V' || $num[$x] == 'L' || $num[$x] == 'D')
      $sub = false;
    $lastCount = 1;
    $lastVal = $val;

    if ($sub) {
      if ($lastGlyph === 'I' && $num[$x] !== 'V' && $num[$x] !== 'X')
        throw new \Exception("Invalid roman numeral.");
      if ($num[$x] !== 'L' && $num[$x] !== 'M' && $num[$x] !== 'C' && $lastGlyph == 'X')
        throw new \Exception("Invalid roman numeral.");
    }
    $lastGlyph = $num[$x];
  }

  if (ROMAN_VERBOSE === true) {
    echo "********* LEAVING x = ".$x." *********".PHP_EOL;
    echo "Value: ".$val.PHP_EOL;
    echo "Last Count: ".$lastCount.PHP_EOL;
    echo "Last Value: ".$lastVal.PHP_EOL;
    echo "Glyph: ".$num[$x].PHP_EOL;
    echo "Last Glyph: ".$lastGlyph.PHP_EOL;
    echo "Current Number: ".$nbr.PHP_EOL;
    echo "Subtract: ".($sub !== false ? "Y" : "N").PHP_EOL;
    echo PHP_EOL;
  }
}
return $nbr + ($sub === true ? -1 : 1) * ($lastVal * $lastCount);
}
}

echo RomanNumeral::toNumber("DLXVIII").PHP_EOL;  // should be 568
echo RomanNumeral::toNumber("MMMCMXCIX").PHP_EOL; // should be 3999
echo RomanNumeral::toNumber("DLXIVIII").PHP_EOL;  // throws exception
?>

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在for循环后调用romantoint

def checkIfRomanNumeral(numeral):
     """Controls that the userinput only contains valid roman numerals"""
     numeral = numeral.upper()
     validRomanNumerals = ["M", "D", "C", "L", "X", "V", "I"]
     for letters in numeral:
        if letters not in validRomanNumerals:
            print("Sorry that is not a valid roman numeral")
            return False
     romanToInt(numeral)

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