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, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII

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

• 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. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

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

Example 5:

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

用hashmap存字元到數字的對映，再遍歷string：如果當前字元是最後一個字元，加上當前字元；如果下一個字元<=當前字元，加上當前字元；否則，減去當前字元

time: O(n), space: O(n)

class Solution {
    public int romanToInt(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        map.put('I', 1);
        map.put('V', 5);
        map.put('X', 10);
        map.put('L', 50);
        map.put('C', 100);
        map.put('D', 500);
        map.put('M', 1000);
        
        int res = 0;
        for(int i = 0; i < s.length(); i++) {
            if(i == s.length() - 1) {
                res += map.get(s.charAt(i));
            } else if(map.get(s.charAt(i+1)) <= map.get(s.charAt(i))){
                res += map.get(s.charAt(i));
            } else {
                res -= map.get(s.charAt(i));
            }
        }
        return res;
    }
}