romanToInt.java

/*
Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000
For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

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.
Given a roman numeral, convert it to an integer.

 

Example 1:

Input: s = "III"
Output: 3
Example 2:

Input: s = "IV"
Output: 4
Example 3:

Input: s = "IX"
Output: 9
Example 4:

Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.
Example 5:

Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
 

Constraints:

1 <= s.length <= 15
s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
It is guaranteed that s is a valid roman numeral in the range [1, 3999].
*/


// functional code

class Solution {
    public int romanToInt(String s) {
        
		//Create HashMap to insert all the necessary roman numeral and its number
       HashMap<Character, Integer> ht = new HashMap<Character, Integer>();
        
		//Now insert it
        ht.put('M', 1000);
        ht.put('D', 500);
        ht.put('C', 100);
        ht.put('L', 50);
        ht.put('X', 10);
        ht.put('V', 5);
        ht.put('I', 1);
        
		//This is to store all the total number of roman numeral integers
        int total = 0;
        
        for(int i = 0; i < s.length() - 1; i++) {
			//In here, if the roman numeral is let's say IV, then we first subtract 1 and add in 5. It's same logic as 5-1 = 4.
            if(ht.get(s.charAt(i)) < ht.get(s.charAt(i + 1))) {
                total -= ht.get(s.charAt(i));
            } else {
				//If not, then simply add the number to total
                total += ht.get(s.charAt(i));
            }
        }
		// Due to out of bound in the last character of the string, we need to add it separately
        return total += ht.get(s.charAt(s.length() - 1));
        
    }
}