Thread: Duplicate elements in 2 Arraylist

Concatenate the two arrays together and try to build a heap. If there's a collision on the comparison of any two strings while building the heap, you know that two strings are lexicographically equal. You can throw out one of the duplicates and continue building the heap to find all duplicates (note that you'll have to take into account the fact that the heap size just decreased), or keep both and use an external list to keep track of duplicates. Alternatively, try to do a merge sort of the two arrays. If you run into two strings being equal during the merge portion, you know there's a duplicate. Remove one of them and continue.

A Collection object contains methods that allow one to find unions, intersections, etc... between two (or more) Collection objects. For example, to find Objects identical between two Collection's (in your case Lists), create a new list, add all items from the first list to this object, then call retainAll(), passing the second list. The result is a list containing items that are found in both.