LeetCode-Roman to Integer

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

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, which is simply X + II. The number twenty seven 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. 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: C = 100, L = 50, XXX = 30 and III = 3.

Example 5:
Input: “MCMXCIV”
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

题意：根据所给的罗马数字组合，输出所得的阿拉伯数字；

解法：首先，我们可以用哈希表来存储罗马数字与阿拉伯数字的映射关系，这样可以直接根据键得到值；其中，有三种情况需要注意

1. IV和IX形式的罗马数字组合，需要减1
2. XL和XC形式的罗马数字组合，需要减10
3. CD和CM形式的罗马数字组合，需要减100

我们可以从所给字符串的尾部开始计算，判断当前是否符合上述三种情况之一，再做相应的处理；

class Solution {
    public int romanToInt(String s) {
        //建立哈希表存储罗马数字与阿拉伯数字的映射关系
        Map<Character, Integer> table = new HashMap<>();
        table.put('I', 1);
        table.put('V', 5);
        table.put('X', 10);
        table.put('L', 50);
        table.put('C', 100);
        table.put('D', 500);
        table.put('M', 1000);

        int num = 0;
        for(int i = s.length() - 1; i >= 0; i--){
            if(s.charAt(i) == 'I' && i < s.length() - 1 && (s.charAt(i+1) == 'V' || s.charAt(i+1) == 'X')){
                num -= table.get(s.charAt(i));
            }//IV、IX
            else if(s.charAt(i) == 'X' && i < s.length() - 1 && (s.charAt(i+1) == 'L' || s.charAt(i+1) == 'C')){
                num -= table.get(s.charAt(i));
            }//XL、XC
            else if(s.charAt(i) == 'C' && i < s.length() - 1 && (s.charAt(i+1) == 'D' || s.charAt(i+1) == 'M')){
                num -= table.get(s.charAt(i));
            }//CD、CM
            else{
                num += table.get(s.charAt(i));
            }
        }
        return num;
    }
}