Roman to Integer

2018-07-25

13. Roman to Integer

Difficulty: Easy

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, 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:

1 2	Input: "III" Output: 3

Example 2:

1 2	Input: "IV" Output: 4

Example 3:

1 2	Input: "IX" Output: 9

Example 4:

1
2
3

**Input:** "LVIII"
**Output:** 58
**Explanation:** C = 100, L = 50, XXX = 30 and III = 3.

Example 5:

1
2
3

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

Solution

从前往后扫描，用一个临时变量记录分段数字。
如果当前比前一个大，说明这一段的值应该是当前这个值减去上一个值。比如 IV = 5 – 1；否
则，将当前值加入到结果中，然后开始下一段记录。比如 VI = 5 + 1, II=1+1

Language: Java

class Solution {
    
    public int map(Character c){
        switch(c){
            case 'I': return 1;
            case 'V': return 5;
            case 'X': return 10;
            case 'L': return 50;
            case 'C': return 100;
            case 'D': return 500;
            case 'M': return 1000;
            default:return 0;
        }
    }

    public int romanToInt(String s) {
        int number = 0;
        int last = 0;
        for(int i=0;i<s.length();++i){
            Character c = s.charAt(i);
            int cur = map(c);
            if(cur>last){
                number+= cur - 2*last;
            }else{
                number+=cur;
            }
            last = cur;

        }
        return number;
    }
}