Tree Traversal Preorder Root Left Right in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the order of visiting nodes in Preorder Tree Traversal?

In Preorder traversal, nodes are visited in this order: Root first, then Left subtree, and finally Right subtree.

intermediate

Why is Preorder traversal useful in copying a tree?

Preorder traversal visits the root before its children, so it can recreate the tree structure by copying the root node first, then recursively copying left and right subtrees.

intermediate

Rust code snippet: What does this function do?

fn preorder(node: &Option<Box<Node>>) {
  if let Some(n) = node {
    println!("{}", n.value);
    preorder(&n.left);
    preorder(&n.right);
  }
}

This function performs a Preorder traversal on a binary tree node: it prints the current node's value, then recursively visits the left child, then the right child.

beginner

In Preorder traversal, what happens if a node has no left child?

If a node has no left child, the traversal skips the left subtree and moves directly to visit the right subtree after visiting the root.

beginner

What is the output of Preorder traversal on this tree? 1 / \ 2 3 / / \ 4 5 6

The Preorder traversal visits nodes in Root-Left-Right order: 1 -> 2 -> 4 -> 3 -> 5 -> 6

In Preorder traversal, which node is visited first?

A Root node

B Left child

C Right child

D Leaf node

Preorder traversal always visits the root node first before its children.

What is the correct order of nodes visited in Preorder traversal?

A Left, Root, Right

B Root, Left, Right

C Left, Right, Root

D Right, Left, Root

Preorder traversal visits nodes in Root, then Left subtree, then Right subtree order.

Which traversal is best to use if you want to copy a tree exactly as it is?

A Inorder

B Postorder

C Level order

D Preorder

Preorder traversal visits the root before children, making it ideal for copying the tree structure.

In Rust, how do you check if a tree node exists before visiting it in Preorder traversal?

A Both B and C

B if let Some(n) = node {}

C match node { None => {}, _ => {} }

D if node.is_none() {}

Both 'if let Some(n) = node' and 'match node' can be used to check node existence in Rust.

What will be printed first in Preorder traversal of a tree with root value 10?

A Right child value

B Left child value

C 10

D Nothing

Preorder traversal prints the root node value first.

Explain the steps of Preorder traversal on a binary tree.

Think about the order Root, Left, Right.

Write a simple Rust function to perform Preorder traversal on a binary tree and print node values.

Use Option and recursion in Rust.

Practice

1. What is the order of nodes visited in a preorder tree traversal?

easy

A. Root, Left, Right

B. Left, Root, Right

C. Left, Right, Root

D. Right, Root, Left

Solution

1. Step 1: Understand preorder traversal definition

   Preorder traversal visits the root node first, then the left subtree, and finally the right subtree.

2. Step 2: Match the order with options

   Root, Left, Right matches the order Root, Left, Right exactly.

3. Final Answer:

   Root, Left, Right -> Option A

4. Quick Check:

   Preorder = Root, Left, Right [OK]

Hint: Remember preorder starts with the root node first [OK]

Common Mistakes:

• Confusing preorder with inorder or postorder
• Swapping left and right subtree order
• Thinking traversal starts with left child

2. Which Rust code snippet correctly implements preorder traversal for a binary tree node Node with left and right children?

easy

A. fn preorder(node: &Option) { if let Some(n) = node { print!("{} ", n.value); preorder(&n.left); preorder(&n.right); } }

B. fn preorder(node: &Option) { if let Some(n) = node { preorder(&n.left); print!("{} ", n.value); preorder(&n.right); } }

C. fn preorder(node: &Option) { if let Some(n) = node { preorder(&n.left); preorder(&n.right); print!("{} ", n.value); } }

D. fn preorder(node: &Option) { if let Some(n) = node { preorder(&n.right); print!("{} ", n.value); preorder(&n.left); } }

Solution

1. Step 1: Identify preorder traversal steps in code

   Preorder visits root first (print value), then left subtree, then right subtree.

2. Step 2: Check each option's order

   fn preorder(node: &Option) { if let Some(n) = node { print!("{} ", n.value); preorder(&n.left); preorder(&n.right); } } prints root value first, then recursively visits left and right children, matching preorder.

3. Final Answer:

   Print root value first, then left, then right -> Option A

4. Quick Check:

   Print root before children = preorder [OK]

Hint: Print root value before recursive calls for preorder [OK]

Common Mistakes:

• Printing after left subtree (inorder)
• Printing after both subtrees (postorder)
• Swapping left and right recursive calls

3. Given the binary tree below, what is the preorder traversal output?

    10
   /  \
  5    15
 /    /  \
3    12  20

medium

A. [10, 15, 20, 12, 5, 3]

B. [3, 5, 10, 12, 15, 20]

C. [5, 3, 10, 12, 15, 20]

D. [10, 5, 3, 15, 12, 20]

Solution

1. Step 1: Start at root and visit nodes in preorder

   Visit root 10, then left subtree: 5, then 3. Then right subtree: 15, then 12, then 20.

2. Step 2: Write traversal sequence

   Sequence is 10, 5, 3, 15, 12, 20 matching [10, 5, 3, 15, 12, 20].

3. Final Answer:

   [10, 5, 3, 15, 12, 20] -> Option D

4. Quick Check:

   Preorder visits root first, then left, then right [OK]

Hint: Follow root-left-right order step by step [OK]

Common Mistakes:

• Listing nodes in inorder or postorder
• Mixing left and right subtree order
• Skipping leaf nodes

4. The following Rust function is intended to perform preorder traversal but has a bug. What is the bug?

fn preorder(node: &Option) {
    if let Some(n) = node {
        preorder(&n.left);
        print!("{} ", n.value);
        preorder(&n.right);
    }
}

medium

A. It visits the right subtree before the left subtree.

B. It does not check if the node is None before recursion.

C. It prints the root value after visiting the left subtree, so it is inorder traversal.

D. It does not print the root value at all.

Solution

1. Step 1: Analyze order of operations in function

   The function calls preorder on left child first, then prints root value, then calls preorder on right child.

2. Step 2: Identify traversal type from order

   This order is Left, Root, Right which is inorder traversal, not preorder.

3. Final Answer:

   Print happens after left subtree, so it's inorder traversal -> Option C

4. Quick Check:

   Preorder prints root first, this prints after left [OK]

Hint: Print root before recursive calls for preorder [OK]

Common Mistakes:

• Confusing preorder with inorder traversal
• Ignoring order of print statement
• Assuming recursion order alone defines traversal

5. You want to collect all node values in preorder traversal into a vector in Rust. Which code snippet correctly does this?

hard

A. fn preorder_collect(node: &Option, vals: &mut Vec<i32>) { if let Some(n) = node { preorder_collect(&n.left, vals); vals.push(n.value); preorder_collect(&n.right, vals); } }

B. fn preorder_collect(node: &Option, vals: &mut Vec<i32>) { if let Some(n) = node { vals.push(n.value); preorder_collect(&n.left, vals); preorder_collect(&n.right, vals); } }

C. fn preorder_collect(node: &Option, vals: &mut Vec<i32>) { if let Some(n) = node { preorder_collect(&n.left, vals); preorder_collect(&n.right, vals); vals.push(n.value); } }

D. fn preorder_collect(node: &Option, vals: &mut Vec<i32>) { if let Some(n) = node { preorder_collect(&n.right, vals); vals.push(n.value); preorder_collect(&n.left, vals); } }

Solution

1. Step 1: Understand preorder collection order

   Preorder means add root value first, then collect left subtree, then right subtree.

2. Step 2: Check which code pushes root value first

   fn preorder_collect(node: &Option, vals: &mut Vec<i32>) { if let Some(n) = node { vals.push(n.value); preorder_collect(&n.left, vals); preorder_collect(&n.right, vals); } } pushes root value before recursive calls, matching preorder.

3. Final Answer:

   Push root value first, then left, then right -> Option B

4. Quick Check:

   Push root before children for preorder collection [OK]

Hint: Push root value before recursive calls to collect preorder [OK]

Common Mistakes:

• Pushing after left subtree (inorder)
• Pushing after both subtrees (postorder)
• Swapping left and right recursion order