13. Roman to Integer

Question

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 ones 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.

Solution

倒叙遍历，记录几个mark以标注是否出现对应的字母。

初始res为0，根据字符加减数值。
如果出现则将mark设置为真。
如果mark为真，且上一个字符出现对应需要减去的罗马字符，则改加为减。

最后返回res值。

Code

class Solution {
    public int romanToInt(String s) {
        int res = 0;
        boolean markX = false, markC = false, markM = false;
        
        for(int i = s.length()-1; i >= 0; i--){
            char c = s.charAt(i);
            if(c == 'I'){
                if(markX) res -= 1;
                else res += 1;
            }
            else if(c == 'V'){
                markX = true;
                res += 5;
            }
            else if(c == 'X'){
                markX = true;
                if(markC) res -= 10;
                else res += 10;
            }
            else if(c == 'L'){
                markC = true;
                res += 50;
            }
            else if(c == 'C'){
                markC = true;
                if(markM) res -= 100;
                else res += 100;
            }
            else if(c == 'D'){
                markM = true;
                res += 500;
            }
            else if(c == 'M'){
                markM = true;
                res += 1000;
            }
        }
        return res;
    }
}