Roman to Integer

Problem:

Given a roman numeral, convert it to an integer.

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

Analysis:

The tricky in this problem is that Symbols are placed from left to right in order of value, starting with the largest. However, in a few specific cases, to avoid four characters being repeated in succession (such as IIII or XXXX) these can be reduced using subtractive notation as follows:

• the numeral I can be placed before V and X to make 4 units (IV) and 9 units (IX respectively)
• X can be placed before L and C to make 40 (XL) and 90 (XC respectively)
• C can be placed before D and M to make 400 (CD) and 900 (CM) according to the same pattern

An example using the above rules would be 1904: this is composed of 1 (one thousand), 9 (nine hundreds), 0 (zero tens), and 4 (four units). To write the Roman numeral, each of the non-zero digits should be treated separately. Thus 1,000 = M, 900 = CM, and 4 =IV. Therefore, 1904 is MCMIV.

The solution is that add up every character, if the previous character is smaller than the  current character then subtract the previous character twice. 

Solution:

1:  public class Solution {  

2:    public int romanToInt(String s) {  

3:      if(s==null) return 0;  

4:      if(s.length()==1) return getInt(s.charAt(0));  

5:      int pre=s.charAt(0);  

6:      int cur=0;  

7:      int total=s.charAt(0);  

8:      for(int i=1;i<s.length();i++)  

9:      {  

10:        cur=getInt(s.charAt(i));  

11:        total+=cur;  

12:        if(pre<cur)  

13:          total=total-pre*2;  

14:        pre=cur;  

15:      }  

16:      return total;  

17:    }  

18:    public int getInt(char digit)  

19:    {  

20:      int result=0;  

21:      switch (digit)  

22:      {  

23:        case 'I': result=1; break;  

24:        case 'V': result=5; break;  

25:        case 'X': result=10;break;  

26:        case 'L': result=50;break;  

27:        case 'C': result=100;break;  

28:        case 'D': result=500;break;  

29:        case 'M': result=1000;break;  

30:      }  

31:      return result;  

32:    }  

33:  }  

Reference:
http://en.wikipedia.org/wiki/Roman_numerals