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

Recall & Review

beginner

What is preorder tree traversal?

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

beginner

In preorder traversal, which node is visited first?

The root node is visited first in preorder traversal.

beginner

What is the order of visiting nodes in preorder traversal?

The order is Root -> Left subtree -> Right subtree.

intermediate

How does preorder traversal differ from inorder traversal?

Preorder visits root first, then left and right. Inorder visits left subtree first, then root, then right subtree.

beginner

Write the preorder traversal output for this tree:
Root: 1
Left child: 2
Right child: 3

The preorder traversal visits nodes in this order: 1 -> 2 -> 3

In preorder traversal, which node is visited second?

A Right child of root

B Leaf node

C Root node

D Left child of root

After visiting the root, preorder traversal visits the left subtree first.

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

A Left, Right, Root

B Left, Root, Right

C Root, Left, Right

D Right, Left, Root

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

Which traversal visits the root node first?

A Preorder

B Inorder

C Postorder

D Level order

Preorder traversal always visits the root node first.

If a tree has only one node, what is the preorder traversal output?

A The single node

B Empty

C Null

D Left child

With one node, preorder traversal visits that node first and only.

Which traversal method is NOT preorder?

A Root, Left, Right

B Left, Root, Right

C thgiR ,tfeL ,tooR

D oot, Left, Right

Left, Root, Right is inorder traversal, not preorder.

Explain how preorder traversal works on a binary tree.

Think about the order you would read a family tree starting from the top.

Describe the difference between preorder and inorder traversal.

Focus on when the root node is visited in each traversal.

Practice

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

easy

A. Left, Right, Root

B. Left, Root, Right

C. Root, Left, Right

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: Confirm the visiting order

   The visiting order is Root, then Left child, then Right child.

3. Final Answer:

   Root, Left, Right -> Option C

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
• Visiting left or right child before root
• Mixing up left and right subtree order

2. Which of the following is the correct JavaScript function signature for preorder traversal of a binary tree node named root?

easy

A. function preorder(root) { if(root) { preorder(root.left); preorder(root.right); console.log(root.value); } }

B. function preorder(root) { if(root) { preorder(root.left); console.log(root.value); preorder(root.right); } }

C. function preorder(root) { if(root) { preorder(root.right); preorder(root.left); console.log(root.value); } }

D. function preorder(root) { if(root) { console.log(root.value); preorder(root.left); preorder(root.right); } }

Solution

1. Step 1: Recall preorder traversal order

   Preorder visits root first, then left subtree, then right subtree.

2. Step 2: Check function steps

   function preorder(root) { if(root) { console.log(root.value); preorder(root.left); preorder(root.right); } } prints root value first, then recursively visits left and right children, matching preorder.

3. Final Answer:

   function preorder(root) { if(root) { console.log(root.value); preorder(root.left); preorder(root.right); } } -> Option D

4. Quick Check:

   Print root before children = function preorder(root) { if(root) { console.log(root.value); preorder(root.left); preorder(root.right); } } [OK]

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

Common Mistakes:

• Printing after visiting children (postorder)
• Swapping left and right recursive calls
• Missing base case for null root

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

    1
   / \
  2   3
 / \
4   5

Use the format: 1 -> 2 -> 4 -> 5 -> 3 -> null

medium

A. 1 -> 2 -> 4 -> 5 -> 3 -> null

B. 4 -> 2 -> 5 -> 1 -> 3 -> null

C. 2 -> 4 -> 5 -> 1 -> 3 -> null

D. 1 -> 3 -> 2 -> 4 -> 5 -> null

Solution

1. Step 1: Start at root node 1

   Visit root first: 1

2. Step 2: Traverse left subtree of 1

   Visit 2, then its left child 4, then right child 5

3. Step 3: Traverse right subtree of 1

   Visit 3

4. Final Answer:

   1 -> 2 -> 4 -> 5 -> 3 -> null -> Option A

5. Quick Check:

   Preorder visits root, left, right = 1 -> 2 -> 4 -> 5 -> 3 -> null [OK]

Hint: Follow root, then left subtree, then right subtree order [OK]

Common Mistakes:

• Visiting left subtree after right subtree
• Starting traversal from left child instead of root
• Confusing preorder with inorder or postorder

4. The following JavaScript code is intended to perform a preorder traversal and print node values. What is the error?

function preorder(root) {
  if (root === null) return;
  preorder(root.left);
  console.log(root.value);
  preorder(root.right);
}

medium

A. The console.log should be before the recursive calls

B. The base case should check for undefined, not null

C. The function should return a value instead of void

D. The left and right recursive calls are swapped

Solution

1. Step 1: Analyze the order of operations

   The code visits left child first, then prints root, then visits right child.

2. Step 2: Compare with preorder traversal order

   Preorder requires printing root before visiting children, so console.log should be before recursive calls.

3. Final Answer:

   The console.log should be before the recursive calls -> Option A

4. Quick Check:

   Print root first in preorder [OK]

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

Common Mistakes:

• Printing after left subtree instead of before
• Confusing preorder with inorder traversal
• Ignoring base case for null nodes

5. You have a binary tree where some nodes have only one child. How does preorder traversal handle nodes with a missing left child?
Consider this tree:

    10
      \
      20
     /  \
   30    40

What is the preorder traversal output?

hard

A. 10 -> 40 -> 20 -> 30 -> null

B. 10 -> 20 -> 30 -> 40 -> null

C. 10 -> 20 -> 40 -> 30 -> null

D. 10 -> 30 -> 20 -> 40 -> null

Solution

1. Step 1: Visit root node 10

   Start with root: 10

2. Step 2: Check left child of 10

   Left child is missing, so skip to right child 20

3. Step 3: Visit node 20 and its children

   Visit 20, then left child 30, then right child 40

4. Final Answer:

   10 -> 20 -> 30 -> 40 -> null -> Option B

5. Quick Check:

   Preorder visits root, left (if any), right = 10 -> 20 -> 30 -> 40 -> null [OK]

Hint: Skip missing children, visit existing nodes in preorder order [OK]

Common Mistakes:

• Assuming missing left child means skip right child
• Visiting children in wrong order
• Confusing preorder with other traversals