[Leetcode 17] 13 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:

Some Roman Number examples can be seen here: http://literacy.kent.edu/Minigrants/Cinci/romanchart.htm. 

Use a one pass scan to decide whether add or subtract a number from current sum.

If a number is greater or equal to its successor or it's the last number, add it.

If a numver is less than its successor, subtract it.

Code:

public class Solution {
    public int romanToInt(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int sum = 0;
        
        for (int i=0; i<s.length(); ++i) {
            int numi = rtoi(s.charAt(i));
            
            if (i==(s.length()-1)) {
                sum += numi;
            } else if ( numi >= rtoi(s.charAt(i+1))) {
                sum += numi;
            } else {
                sum -= numi;
            }
        }
    
        return sum;
    }
    
    private int rtoi(char x) {
        int val=0; 
        
        switch (x) {
            case 'I': val = 1; break;
            case 'V': val = 5; break;
            case 'X': val = 10; break;
            case 'L': val = 50; break;
            case 'C': val = 100; break;
            case 'D': val = 500; break;
            case 'M': val = 1000; break;
        }
        
        return val;
    }
}

An efficient version that call helper function only once in each iteration

public class Solution {
    public int romanToInt(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int sum = 0, numi=0, numiSucc=rtoi(s.charAt(0));
        
        for (int i=0; i<s.length(); ++i) {
            numi = numiSucc;
            
            if (i==(s.length()-1)) {
                sum += numi;
                break;
            }
            
            numiSucc = rtoi(s.charAt(i+1));
            if ( numi >= numiSucc) {
                sum += numi;
            } else {
                sum -= numi;
            }
        }
    
        return sum;
    }
    
    private int rtoi(char x) {
        int val=0; 
        
        switch (x) {
            case 'I': val = 1; break;
            case 'V': val = 5; break;
            case 'X': val = 10; break;
            case 'L': val = 50; break;
            case 'C': val = 100; break;
            case 'D': val = 500; break;
            case 'M': val = 1000; break;
        }
        
        return val;
    }
}

Attention:

Know what is Roman Number before coding.