romanToInt.js

//input string
//output number

// 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.

var romanToInt = function(string) {
  var reference = {
    'I': 1,
    'V': 5,
    'X': 10,
    'L': 50,
    'C': 100,
    'D': 500,
    'M': 1000,
  };
  var result = 0;
  for (var i = 0; i < string.length; i++) {
    if (string[i] == 'I' && (string[i+1] == 'V' || string[i+1] == 'X')) {
      result = result + (reference[string[i+1]] - reference[string[i]]);
      i++;
    } else if (string[i] == 'X' && (string[i+1] == 'L' || string[i+1] == 'C')) {
      result = result + (reference[string[i+1]] - reference[string[i]]);
      i++;
    } else if (string[i] == 'C' && (string[i+1] == 'D' || string[i+1] == 'M')) {
      result = result + (reference[string[i+1]] - reference[string[i]]);
      i++;
    } else {
      result = result + reference[string[i]];
    }
  }
  return result;
};

romanToInt('LVIII');