followed along

src/Traversal.java

@@ -1,5 +1,139 @@
+import static org.junit.jupiter.api.DynamicTest.stream;
+
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.Map;
+import java.util.Queue;
+import java.util.Stack;
+
 public class Traversal {
   public static void main(String[] args) {
+    TreeNode<Integer> root = new TreeNode<Integer>(10, null, null);
+    root.left = new TreeNode<Integer>(9, null, null);
+    root.left.left = new TreeNode<Integer>(5, null, null);
+    root.left.right = new TreeNode<Integer>(2, null, null);
+
+    root.right = new TreeNode<Integer>(15, null, null);
+    root.right.left = new TreeNode<Integer>(-3, null, null);
+    root.right.right = new TreeNode<Integer>(5, null, null);
+    root.right.right.right = new TreeNode<Integer>(22, null, null);
+
+    TreeNode<String> stringRoot = new TreeNode<String>("hello", null, null);
+    stringRoot.left = new TreeNode<String>("How", null, null);
+    stringRoot.left.left = new TreeNode<String>("and", null, null);
+    stringRoot.left.right = new TreeNode<String>("and", null, null);
+
+    stringRoot.right = new TreeNode<String>("I", null, null);
+    stringRoot.right.left = new TreeNode<String>("am", null, null);
+    stringRoot.right.right = new TreeNode<String>("so", null, null);
+    stringRoot.right.right.right = new TreeNode<String>("good", null, null);
+
+    TreeNode<Integer> megaRoot = new TreeNode<Integer>(1, null, null);
+    TreeNode<Integer> current = megaRoot;
+    for(int i = 2; i <= 50000; i++) {
+      current.right = new TreeNode<Integer>(i, null, null);
+      current = current.right;
+    }
+
+    levelOrderIter(root);
+
+    //preOrderIter(megaRoot);
+    // System.out.println("Preorder Recursive");
+    // preOrder(root);
+
+    // System.out.println("Preorder Iterative");
+    // preOrderIter(root);
+
+    // preOrder(root);
+    System.out.println();
+    // postOrder(root);
+    //printGrader(root, 100);
+    // System.out.println(countNodes(stringRoot));
+
+    //System.out.println(toMap(stringRoot));
+  }
+
+  public static <T> void preOrderIter(TreeNode<T> node) {
+    Stack<TreeNode<T>> stack = new Stack<>();
+    stack.push(node);
+    while(!stack.isEmpty()) {
+      TreeNode<T> currentNode = stack.pop();
+
+      if(currentNode == null) continue; // move onto next iteration, will not stop loop
+
+      System.out.println(currentNode.value);
+      stack.push(currentNode.right);
+      stack.push(currentNode.left);
+    } 
+  }
+
+  public static <T> void levelOrderIter(TreeNode<T> node) {
+    Queue<TreeNode<T>> queue = new LinkedList<>();
+    queue.add(node);
+    while(!queue.isEmpty()) {
+      TreeNode<T> currentNode = queue.remove();
+
+      if(currentNode == null) continue; // move onto next iteration, will not stop loop
+
+      System.out.println(currentNode.value);
+      queue.add(currentNode.left);
+      queue.add(currentNode.right);
+    } 
+  }
+
+  // Print tree rooted at the given node in pre-order.
+  // ? if you dont need to reuse it, parameterize
+  public static void preOrder(TreeNode<?> node) {
+    if (node == null)
+      return; 
+    System.out.println(node.value);
+    preOrder(node.left);
+    preOrder(node.right);
+  }
+
+  public static <T> Map<T, Integer> toMap(TreeNode<T> node) {
+    Map<T, Integer> counts = new HashMap<>();
+    toMap(node, counts);
+    return counts;
+  }
+
+  private static <T> void toMap(TreeNode<T> node, Map<T, Integer> counts) {
+    if(node == null) return;
+    // fill up those counts
+    counts.put(node.value, counts.getOrDefault(node.value, 0) + 1);
+    toMap(node.left, counts);
+    toMap(node.right, counts);
+  }
+
+  public static int countNodes(TreeNode<?> node) {
+    return node == null ? 0 : 1 + countNodes(node.left) + countNodes(node.right);
+  }
+
+  public static void printGrader(TreeNode<Integer> node, int threshold) {
+    if (node == null) return;
+    if(node.value > threshold) {
+      System.out.println(node.value);
+    }
+    printGrader(node.left, threshold);
+    printGrader(node.right, threshold);
+  }
+
+
+
+  // type variable if you need to reuse it
+  public static <T> void postOrder(TreeNode<T> node) {
+    if (node == null)
+      return;
+    postOrder(node.left);
+    postOrder(node.right);
+    System.out.println(node.value);
+  }
 
+  public static <E> void inOrder(TreeNode<E> node) {
+    if (node == null)
+      return;
+    inOrder(node.left);
+    System.out.println(node.value);
+    inOrder(node.right);
   }
 }

src/TreeNode.java

@@ -1,4 +1,12 @@
-public class TreeNode {
+public class TreeNode<T>{
+    public T value;
+    public TreeNode<T> left;
+    public TreeNode<T> right;
 
+    public TreeNode(T value, TreeNode<T> left, TreeNode<T> right) {
+        this.value = value;
+        this.left = left;
+        this.right = right;
+    }
 
 }