Lecture 10: Trees

October 10, 2016

data structures |

Starting this lecture at 6:40 btw. 30 minutes of nothingness before.


  • Trees are a bunch of nodes
  • Need to have references to child nodes
  • Very likely going to iterate across children, not necessarily jump to a specific one.
  • You could do this with a LinkedList but there’s an easier way
  • To iterate through children, go to firstChild and then iterate through using nextSibling
class TreeNode<T> {
    T data;
    TreeNode<T> nextSibling;  // Similar to LinkedList
    TreeNode<T> firstChild;  // Another LinkedList type structure to children

Pre-order traversal: evaluate the node first, then evaluate the children. Post-order: evaluate the children first, then the node.

Tree Traversal

A pre-order tree traversal via listAll

// Pseudo code
private void listAll(int depth) {
    printName(depth);  // Print name of object
    if(isDirectory()) {
        for each file c in this directory (for each child) {
            c.listAll(depth + 1);

public void listAll() {
    listAll(0);  // Initial call for root node calls actual recursive method

Tree Types

  • Full Binary Tree
    • ?
  • Complete Binary Tree
    • Last row is filled up from left to right but not all the way
  • Perfect Binary Tree
    • A complete binary tree with the last row all filled up

Need to think about the number of nodes in a tree given the height..

A perfect binary tree has 2^(h + 1) - 1 nodes.

Btw, know what “proof by induction” means.

Proof by Induction

Base Case

  • 2^(0 + 1) - 1 = 1
  • 2^(1 + 1) - 1 = 3

Assume for perfect trees of height 1 through k this holds.

So now we need to prove that this works for a tree of height k + 1.

A tree of height k will have 2^(k + 1) - 1 nodes.

Now copy that tree and we have two identical trees.

Now add a root node and point to the root nodes of those two initial trees.

So now we just need to add up the nodes.

2 * (2^(k + 1) - 1) + 1 2^((k + 1) + 1) - 2 + 1 2^((k + 1) + 1) - 1



Expression Tree

Tree will be represented by a full binary tree. In a binary tree, there is something on the “left” and something on the “right” so there is some implicit order.

Operators are interior nodes. Operands are leaf nodes.


3 + (2 * 5)

+ 3 * 2 5

Does the root node of an expression tree have to be an operand? Nope.

Interior nodes must be operators.

Leaf nodes must be operands?

Look at the ExpressionNode class…

public class ExprNode {
    Op data;  // either operator or operand
    ExprNode left;
    ExprNode right;
Alex Scott If you're a nerd, I'm a nerd.
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