13 罗马数字转整数

 public int romanToInt(String s) {
    int[] a = new int[26];
    a['I' - 'A'] = 1;
    a['V' - 'A'] = 5;
    a['X' - 'A'] = 10;
    a['L' - 'A'] = 50;
    a['C' - 'A'] = 100;
    a['D' - 'A'] = 500;
    a['M' - 'A'] = 1000;
    char prev = 'A';
    int sum = 0;
    for(char c : s.toCharArray()) {
        if(a[c - 'A'] > a[prev - 'A']) {    //如果前面的比后面的值小
            sum = sum - 2 * a[prev - 'A'];
        }
        sum = sum + a[c - 'A'];
        prev = c;
    }
    return sum;
}