Leetcode 13. Roman to Integer

Description:

Given a roman numeral, convert it to an integer.
Input is guaranteed to be within the range from 1 to 3999.

Thinking:

We may find from the chart that:

• I should be followed by oneV/X(sum >= 5) or several I(sum < 5);
• X should be followed by one L/C/D/M(sum >= 50) or several I/X(sum < 50);
• C should be followed by one D/M(sum >= 500) or several I/X/L/C(sum < 500);

Steps:

1. Check whether input is valid;
2. Start from the end of the String, check each character and use switch-case structure.
3. When character is I, X, C, we should check the result we already got to decide whether we need to add or subtract.

Code:

public class Solution {
    public int romanToInt(String s) {
        if(s.isEmpty()) return -1;
        int result = 0;
        for(int i = s.length()-1; i >=0; i--){
            char roman = s.charAt(i);
            switch(roman){
                case 'I':
                    result += result >= 5 ? -1 : 1;
                    break;
                case 'V':
                    result += 5;
                    break;
                case 'X':
                    result += result >= 50 ? -10 : 10;
                    break;
                case 'L':
                    result += 50;
                    break;
                case 'C':
                    result += result >= 500 ? -100 : 100;
                    break;
                case 'D':
                    result += 500;
                    break;
                case 'M':
                    result += 1000;
                    break;
               //Also can add default:break;
            }
        }
        return result;

    }
}

Conclusion:

Trying to look for some patterns and rules is one of the best way to solve the problem of string.