FAQ: Trees: Java - Review

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This community-built FAQ covers the “Review” exercise from the lesson “Trees: Java”.

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Pass the Technical Interview with Java

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[janbazant1107978602]

I tried to redo the breadthFirstTraversal to use an iterable/iterator.
This is what I got:

import java.util.LinkedList;
import java.util.Queue;
import java.util.Iterator;

public class Tree {
  public TreeNode root;

  public Tree(TreeNode root) {
    this.root = root;
  }

  public void print() {
    print(this.root, 0);
  }

  public static void print(TreeNode current, int level) {
    String levelMarks = "";
    for (int i = 0; i < level; i++) {
      levelMarks += "-- ";
    }
    System.out.println(levelMarks + current.data);
    for (TreeNode child : current.children) {
      print(child, level + 1);
    }
  }

  public static void depthFirstTraversal(TreeNode current) {
    System.out.print(current.data + " ");
    for (TreeNode child : current.children) {
      depthFirstTraversal(child);
    }
  }

  public void depthFirstTraversal() {
    this.depthFirstTraversal(this.root);
  }

  private class BreadthFirstIterable implements Iterable<TreeNode>, Iterator<TreeNode> {
    Queue<TreeNode> queue;
    TreeNode current;
    
    public BreadthFirstIterable(TreeNode startNode) {
      current = startNode;
      queue = new LinkedList<>();
      queue.add(current);
    }
    public BreadthFirstIterable() {
      this(root);
    }

    public boolean hasNext() {
      return !queue.isEmpty();
    }
    public TreeNode next() {
      TreeNode current = queue.poll();
      queue.addAll(current.children);
      return current;
    }

    @Override
    public Iterator<TreeNode> iterator() {
      return this;
    }
  }

  public void breadthFirstTraversal() {
    BreadthFirstIterable iterable = new BreadthFirstIterable();
    for (TreeNode current : iterable) {
      System.out.print(current.data + " ");
    }
  }
  /*
  // Define breadthFirstTraversal() below
  public void breadthFirstTraversal() {
    TreeNode current = this.root;
    Queue<TreeNode> queue = new LinkedList<>();
    queue.add(current);
    while (!queue.isEmpty()) {
      current = queue.poll();
      System.out.print(current.data + " ");
      queue.addAll(current.children);
    }
  }
  */

  public static void main(String[] args) {
    TreeNode treeRoot = new TreeNode("S");
    TreeNode child1 = new TreeNode("N");
    TreeNode child2 = new TreeNode("O");
    TreeNode grandchild1 = new TreeNode("W");
    TreeNode grandchild2 = new TreeNode("Y");
    treeRoot.addChild(child1);
    treeRoot.addChild(child2);
    child1.addChild(grandchild1);
    child2.addChild(grandchild2);

    Tree tree = new Tree(treeRoot);
    tree.print();

    // Breadth-first traversal below:
    tree.breadthFirstTraversal();

    System.out.println("\n");
    tree.depthFirstTraversal();
  }
}