Sum of Digits of String After Convert

给一个string表示数字, 求n次数字位求和后的结果.

class Solution {
    public int getLucky(String s, int k) {
        StringBuilder sb = new StringBuilder();
        for(char c : s.toCharArray())
            sb.append((c - 'a') + 1);
        s = sb.toString();
        while(k != 0)
        { 
            int sum = 0;
            for(char c : s.toCharArray())
            {
                sum += c - '0';
            }
            s = sum + "";
            k--;
        }
        return Integer.valueOf(s);
    }
}