是否可以使嵌套的for循环小于O（N ^ 2）？

Question

I am working on a Kafka Streaming service using Apache Spark 2.1 and Java 8. I use a nested for loop to populate an ArrayList with Topic/Partition pairs.

Is it possible to reduce this nested for loop from O(N^2) using another methodology?

Here is the code:

    private static List<TopicAndPartition> createTAndPList(List<String> topics, List<String> partitions)
        throws ConsumerException {
    if (topics.size() != partitions.size()) {
        throw new ConsumerException("Cannot create list with unequal number of topics and parititons,");
    }

    List<TopicAndPartition> topicsAndPartitions = new ArrayList<>();
    for (int t = 0; t < topics.size(); t++) {
        for (int p = 0; p < Integer.parseInt(partitions.get(t)); p++) {
            topicsAndPartitions.add(new TopicAndPartition(topics.get(t), p));
        }
    }

    return topicsAndPartitions;
}

FYI: I am prevented from using above Kafka 8 due to powers beyond my control (management.)

Answer 1

With your given code, it does not look possible to reduce the order.

However, there may be two small optimizations you can make.

• Move topics.get(t) out of the inner for loop,

• Do not recalculate the inner for loop termination condition every loop.

   for (int t = 0; t < topics.size(); t++) { String topic = topics.get(t); int count = Integer.parseInt(partitions.get(t)); for (int p = 0; p < count; p++) { topicsAndPartitions.add(new TopicAndPartition(topic, p)); 

You are calling topics.get and Integer.parseInt(partitions.get(t)) t*p times instead of just t times. The change of topics.get() will probably not do much, but moving something out of the inner loop like this is a pretty common optimization.

Finally, do you really need them all in a list? Or can you generate them dynamically where you actually need them?