// JavaScript
// integer to roman
function convertToRoman(num) {
 var decimalValue = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
 var romanNumeral = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
  var ans = "";
  for (var i = 0; i < decimalValue.length; i++) {
    while (decimalValue[i] <= num) {
      num -= decimalValue[i];
      ans += romanNumeral[i];
      
    }
  }
 return ans;
}

convertToRoman(36);

Roman to integer

class Solution {
    public int romanToInt(String s) {
        int decimalValue[] = {1000, 900, 500, 400, 100, 90, 50, 40,10, 9, 5, 4, 1};
        String romanNumerals[] = {"M", "CM", "D", "CD", "C", "XC", "L", "XL","X","IX", "V", "IV","I"};
        int ans = 0;
        int i = s.length() - 1;
        while (i >= 0) {
            switch(s.charAt(i)) {
                case 'I':
                    ans += 1;
                    i--;
                    break;
                case 'V':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'I') {
                        ans += 4;
                        i -= 2;
                    } else {
                        ans += 5;
                        i--;
                    }
                    break;
                case 'X':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'I') {
                        ans += 9;
                        i -= 2;
                    } else {
                        ans += 10;
                        i--;
                    }
                    break;
                case 'L':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'X') {
                        ans += 40;
                        i -= 2;
                    } else {
                        ans += 50;
                        i--;
                    }
                    break;
                case 'C':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'X') {
                        ans += 90;
                        i -= 2;
                    } else {
                        ans += 100;
                        i--;
                    }
                    break;
                case 'D':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'C') {
                        ans += 400;
                        i -= 2;
                    } else {
                        ans += 500;
                        i--;
                    }
                    break;
                case 'M':
                    if (i - 1 >= 0 && s.charAt(i - 1) == 'C') {
                        ans += 900;
                        i -= 2;
                    } else {
                        ans += 1000;
                        i--;
                    }
                    break;
            }
        }
        return ans;
    }
}