[lintcode] problem 386 – longest substring with at most k distinct characters

public int (String s, int k) {
    int result = 0;
    int[] count = new int[128];
    int num = 0;
    int start = 0;
    int end = 0;

    while (end < s.length()) {
        int idxEnd = s.charAt(end) - '0';

        if (count[idxEnd] == 0)
            num++;

        count[idxEnd]++;
        end++;

        while (num > k) {
            int idxStart = s.charAt(start) - '0';
            count[idxStart]--;

            if (count[idxStart] == 0)
                num--;

            start++;
        }

        result = Math.max(result, end - start);
    }

    return result;
}