[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;
     }
 }