[leetcode] 13. Roman to Integer

如果某一个字母代表的数字大于上一个字母代表的数字小，那么加上这个数字，否则，减去两倍的前一个数字，然后加上这一位数字。

public class Solution {
    private static char[][] chars = {{'I', 'V'}, {'X', 'L'}, {'C', 'D'}, {'M', 'O'}};
    private static HashMap<Character, Integer> map = null;
    private static void init() {
        map = new HashMap<Character, Integer>();
        for (int i = 0; i < 4; i++) {
            map.put(chars[i][0], (int) Math.pow(10, i));
            map.put(chars[i][1], 5 * (int) Math.pow(10, i));
        }
    }
    public static int romanToInt(String s) {
        if (map == null) {
            init();
        }
        int length = s.length();
        int result = 0;
        boolean flag = true;
        int prev = map.get(s.charAt(0));
        for (int i = 1; i < length; i++) {
            int x = map.get(s.charAt(i));
            if (prev >= x) {
                result += prev;
            } else {
                result -= prev;
            }
            prev = x;
        }
        result += prev;
        return result;
    }
}