遍历数组列表Java

Question

I am reading a list of items from an excel sheet. Each item name has a parent item name defined (as a separate column) from the same list. So this forms a tree hierarchy in which one item is a parent of the other in the same list and there are also multiple children for a parent. The item names are not unique in the whole list. Items are unique only under a particular parent.

The Id for each item is generated while reading from input file. Now I want to get for each item the Id of its parent item.

for a current record being read from the file, the parent item can be present down the line which we have not yet read, and hence we do not know the id of its parent.

when I think of a logic it results in looping through array list and hashmap key sets repeatedly to get it.

I need help to get a better logic to solve this. thanks in advance!

EDIT 1:

Now I have problem in defining the structure of Excel sheet itself. I need it to be user friendly. How can I define a parent for an item record, when the names are not unique? should I force the user to provide excel in the hierarchy order itself so that I can easily identify the parent for a child item?

Answer 1

You could use a simple

HashMap<Parent, List<Child>> to allow multiple values to be stored with each parent key -simply append children as needed to the keyed list. Alternatively, consider using a MultiMap to hold your data.

Answer 2

You can simply read the items one time and create a HashMap of (name, id).
And then iterate on the items again and assign the parent id, getting it from the HashMap.
In this way you will iterate only 2 times on the list of items, which will give it time complexity of O(n).

But if you don't have unique names then it may be possible that you don't find a unique parent for an item, if you only have it's parents name on the item.