[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。

 1 public class Solution {
 2     private static char[][] chars = {{'I', 'V'}, {'X', 'L'}, {'C', 'D'}, {'M', 'O'}};
 3     private static HashMap<Character, Integer> map = null;
 4     private static void init() {
 5         map = new HashMap<Character, Integer>();
 6         for (int i = 0; i < 4; i++) {
 7             map.put(chars[i][0], (int) Math.pow(10, i));
 8             map.put(chars[i][1], 5 * (int) Math.pow(10, i));
 9         }
10     }
11     public static int romanToInt(String s) {
12         if (map == null) {
13             init();
14         }
15         int length = s.length();
16         int result = 0;
17         boolean flag = true;
18         int prev = map.get(s.charAt(0));
19         for (int i = 1; i < length; i++) {
20             int x = map.get(s.charAt(i));
21             if (prev >= x) {
22                 result += prev;
23             } else {
24                 result -= prev;
25             }
26             prev = x;
27         }
28         result += prev;
29         return result;
30     }
31 }