Flatten Nested List Iterator in Java
In Java, working with nested lists containing other lists as elements is common. One useful operation that can be performed on nested lists is to flatten them, which means converting them into a single-level list that contains all of the elements of the original lists in the order in which they appeared.
An iterator is a tool in Java that allows you to traverse a collection of objects, such as a list, sequentially. We can flatten them and iterate over the resulting single-level list by creating a custom iterator for nested lists.
We can use a recursive approach to implement a flattened nested list iterator in Java. First, we need to define a class that represents the iterator. This class should have a constructor that inputs a nested list and initializes the iterator. We must also implement the Iterator interface, which requires defining the hasNext() and next() methods.
Here is an example implementation of a flattened nested list iterator in Java:
FlattenList.java
import java.util.*;
public class FlattenList implements Iterator<Integer> {
private Stack<Iterator<NestedInteger>> stack;
private Integer lastReturned;
public FlattenList (List<NestedInteger> nestedList) {
stack = new Stack<>();
stack.push(nestedList.iterator());
}
@Override
public boolean hasNext() {
while (!stack.isEmpty()) {
if (!stack.peek().hasNext()) {
stack.pop();
} else {
NestedInteger curr = stack.peek().next();
if (curr.isInteger()) {
lastReturned = curr.getInteger();
return true;
} else {
stack.push(curr.getList().iterator());
}
}
}
return false;
}
@Override
public Integer next() {
return lastReturned;
}
public static void main(String[] args) {
List<NestedInteger> nestedList = new ArrayList<>();
nestedList.add(new NestedInteger(1));
nestedList.add(new NestedInteger(Arrays.asList(new NestedInteger(2), new NestedInteger(3))));
nestedList.add(new NestedInteger(4));
FlattenList iterator = new FlattenList (nestedList);
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
class NestedInteger {
private List<NestedInteger> list;
private Integer val;
public NestedInteger(Integer val) {
this.val = val;
}
public NestedInteger(List<NestedInteger> list) {
this.list = list;
}
public boolean isInteger() {
return val != null;
}
public Integer getInteger() {
return val;
}
public List<NestedInteger> getList() {
return list;
}
}
Output
1
2
3
4
Explanation
In this implementation, we use a Stack data structure to keep track of the iterators for each nested list. The constructor initializes the stack with an iterator for the top-level nested list.
The hasNext() method is used to check whether the iterator has more elements. It works by checking the top iterator on the stack. If it has more elements, we check whether the next element is an integer or another nested list. If it's an integer, we return true. If it's another nested list, we push its iterator onto the stack and continue checking.
If the top iterator is empty, we pop it off the stack and continue checking the next iterator on the stack. If we've checked all the iterators and still haven't found an integer element, we return false.
The next() method returns the next integer element in the iterator. We simply call the next() method on the top iterator on the stack and return the integer value.
This class has two constructors: one that takes an integer value and one that takes a list of NestedInteger objects. It also has three methods:
isInteger(): Returns true if the NestedInteger represents an integer value.
getInteger(): Returns the integer value of the NestedInteger, or null if it represents a list.
getList(): Returns the list of NestedInteger objects represented by the NestedInteger, or null if it represents an integer value.
To use the NestedListIterator with a custom NestedInteger class, you can modify the implementation of the hasNext() method to check whether the next element is an integer or a list using the isInteger() method of the NestedInteger class.
The flatten nested list iterator is useful for working with nested lists in Java. By creating a custom iterator, we can flatten nested lists and iterate over them in a sequential manner. The implementation shown here can be easily modified to work with custom NestedInteger classes and can be used in a wide range of applications.
Recursive flattening:
Instead of using an iterator, another approach to flattening a nested list is to use recursion. This involves recursively traversing the list and adding each integer element to a new list. Here's an example implementation:
public List<Integer> flattenList(List<NestedInteger> nestedList) {
List<Integer> flattenedList = new ArrayList<>();
for (NestedInteger ni : nestedList) {
if (ni.isInteger()) {
flattenedList.add(ni.getInteger());
} else {
flattenedList.addAll(flattenList(ni.getList()));
}
}
return flattenedList;
}