Java collection nested List of list

This return value is encountered on Leetcode

 Public class Solution {    public list<list<integer>> levelorder (TreeNode root) {            }} 

List<list<integer>>

That is, the list content is the type of list, the direct use of list<list<integer>> list = new list<list<integer>> () is wrong, because list is the interface, Cannot instantiate (cannot instantiate the type list<list<integer>>).

But if you use

1 list<list<integer>> List = new linkedlist<linkedlist<integer>> ();

An error will be added (cannot convert from linkedlist<linkedlist<integer>> to list<list<integer>>),

The correct approach is to modify it into:

1 New Linkedlist<linkedlist<integer>>(); 3 or 5 New Linkedlist<list<integer>> ();

This is possible, that is, the generic type parameters must be the same.

The following treatment

1 New Arraylist<arraylist<string>>(); 2 or 3 New Linkedlist<linkedlist<string>> ();

It is also possible to do so without referencing the implementation class with the interface class .

Reference

1.Working with a List of Lists in Java

2.How do I initialize a two-dimensional List statically?

3.Initialize list<list<integer>> in Java

Java collection nested List of list