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public String[] reorderLogFiles(String[] logs) {
    if (logs == null || logs.length == 0) return logs;
    int len = logs.length;
    List<String> letterList = new ArrayList<>();
    List<String> digitList = new ArrayList<>();
    for (String log : logs) {
        if (log.split(" ")[1].charAt(0) < 'a') {
            digitList.add(log);
        } else {
            letterList.add(log);
        }
    }
    Collections.sort(letterList, (o1, o2) -> {
        String[] s1 = o1.split(" ");
        String[] s2 = o2.split(" ");
        int len1 = s1.length;
        int len2 = s2.length;
        for (int i = 1; i < Math.min(len1, len2); i++) {
            if (!s1[i].equals(s2[i])) {
                return s1[i].compareTo(s2[i]);
            }
        }
        //tie --> use identifier
        return s1[0].compareTo(s2[0]);
    });

    for (int i = 0; i < len; i++) {
        if (i < letterList.size())
            logs[i] = letterList.get(i);
        else logs[i] = digitList.get(i - letterList.size());
    }
    return logs;
}