如何按前3名对ArrayList中的邮政编码列表进行排序？

Question

Currently, I've got an ArrayList of zip codes which represent car accidents.

Ex:

10005
10002
10003
10005
10004

In this case, I would like to count the highest occurrence of each of these zip codes and sort from highest to lowest. I looked around and many people talked about using a HashMap to do this. Unfortunately, I cannot use a HashMap as per the directions on the homework.

What is the best way to count the frequency of each zip occurring, thus effectively counting the number of accidents per zip code without using a HashMap / Hashtable ?

I've thought about doing arrayOfZip[zipCode] = frequency and then sorting by there, but the problem with that is that the array declared will be unnecessarily huge in memory.

Answer 1

You can sort the list, and then iterate through counting how long the run of each zip code is; then just keep track of the length of the 3 longest runs and their corresponding zip code.

Answer 2

I think Colection Frequency based methods woulb be usuful:

int occurrences = Collections.frequency(zip, "xxxxxx");

Answer 3

Sort the list and then:

int counter;
for(int i=0;i<SizeOfList;i+=counter)
//increment counter by no. of times a zipcode occurred to avoid repetation
    {
        counter=1;
        for(j=i+1;j<SizeOfList;++j)
            { 
                if(zipcode[i]==zipcode[j])//checking one zipcode with all next values in list
                    counter++;
            }
        System.out.print("\nFrequency of " + zipcode[i] + " is " + counter);
//getting the frequency of that zipcode and printing it(can store it if you want) 
//Next it will jump the loop by value of counter and hence
//the next zipcode inline will come into loop and it will continue till end
    }

This will essentially print the frequency of all unique zipcodes. You can also always compare the counter and store the highest three frequency zipcodes.