/**
 * @description:
 *
 *  s  
 *
 *
 *  1
 *
 * s = "bcabc"
 * "abc"
 *  2
 *
 * s = "cbacdcbc"
 * "acdb"
 *
 * 1  ch) {
                    if (num[sb.charAt(sb.length() - 1) - 'a'] > 0) {
                        vis[sb.charAt(sb.length() - 1) - 'a'] = false;
                        sb.deleteCharAt(sb.length() - 1);
                    } else {
                        break;
                    }
                }
                vis[ch - 'a'] = true;
                sb.append(ch);
            }
            num[ch - 'a'] += 1;
        }
        return sb.toString();
    }
}