Java - Optimized way to find duplicates from two ArrayLists/Array

Question

I have two Arrays/ArrayLists of Integer. I want to know the optmized way to find duplicates from two and store into the third one.

Array1 = {1,2,3,6,9,10,15,4};  
Array2 = {4,8,6,5,12,14,1,2,9};  
Result Array= {1,2,3,6,9,10,15,4,8,5,12,14,9}

Regards, Android IT

Answer 1

I would prefer to use Set over HashMap as HashMap need key-value & Sets talks about UNIQUENESS . Sets don't allow duplicates...

Answer 2

Add elements of both arrays into a HashMap where the value is the number of times the element occurs. Then output it to an array.

Answer 3

You mean, a set union?

List<Integer> array1 = Arrays.asList(1,2,3,6,9,10,15,4);
Set<Integer> set1 = new HashSet<Integer>(array1);

List<Integer> array2 = Arrays.asList(4,8,6,5,12,14,1,2,9);
Set<Integer> set2 = new HashSet<Integer>(array2);

set1.addAll(set2);
List<Integer> resultArray = new ArrayList<Integer>(set1);

Now resultArray contains

[1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14, 15]

Answer 4

一种有效的方法是对数组（O（n log n））进行排序，并遍历两个数组中的最低数组，仅在两个数组中都为（O（n））时才选择它，否则将其丢弃。