Left Side View of Binary Tree in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Left Side View of a binary tree?

The left side view of a binary tree is the set of nodes visible when the tree is seen from the left side. It includes the leftmost node at each level of the tree.

beginner

Which tree traversal method is commonly used to find the left side view of a binary tree?

Level order traversal (Breadth-First Search) is commonly used. We visit nodes level by level and pick the first node at each level to form the left side view.

beginner

In the context of left side view, why do we pick the first node at each level during traversal?

Because the first node visited at each level from left to right is the leftmost node visible from the left side of the tree.

beginner

What data structure helps to implement level order traversal efficiently for left side view?

A queue is used to hold nodes of each level in order. It helps process nodes level by level from left to right.

beginner

How does the left side view differ from the right side view of a binary tree?

The left side view shows the leftmost nodes at each level, while the right side view shows the rightmost nodes at each level when viewed from the right side.

What node do we include in the left side view at each level?

A The first node visited at that level from left to right

B The last node visited at that level from left to right

C The root node only

D All nodes at that level

The left side view includes the first node visited at each level from left to right, which is the leftmost node.

Which data structure is best suited for level order traversal?

A Stack

B Hash Map

C Queue

D Linked List

A queue is used to process nodes level by level in order, which is essential for level order traversal.

If a binary tree has only one node, what is its left side view?

A Empty

B The root node

C All nodes except root

D None of the above

With only one node, the left side view is just the root node itself.

What traversal order is used to find the left side view?

A Level order

B Preorder

C Postorder

D Inorder

Level order traversal visits nodes level by level, which helps identify the leftmost node at each level.

How do you handle nodes at the same level when finding the left side view?

A Pick the rightmost node

B Pick the middle node

C Pick all nodes

D Pick the leftmost node

For the left side view, we pick the leftmost node at each level.

Explain how to find the left side view of a binary tree using level order traversal.

Think about how you see the tree from the left side and which nodes appear first at each level.

Describe the difference between left side view and right side view of a binary tree.

Imagine looking at the tree from the left side versus the right side.

Practice

1. What does the Left Side View of a binary tree represent?

easy

A. Nodes visible when the tree is viewed from the right side

B. Nodes at the deepest level of the tree

C. All leaf nodes of the tree

D. Nodes visible when the tree is viewed from the left side

Solution

1. Step 1: Understand the view perspective

   The left side view shows nodes visible when looking at the tree from the left side.

2. Step 2: Identify visible nodes

   Only the first node at each level from the left is visible in this view.

3. Final Answer:

   Nodes visible when the tree is viewed from the left side -> Option D

4. Quick Check:

   Left side view = nodes visible from left [OK]

Hint: Left side view shows first node at each level from left [OK]

Common Mistakes:

• Confusing left side view with right side view
• Thinking it shows all leaf nodes
• Assuming it shows deepest nodes only

2. Which of the following TypeScript function signatures correctly represents a function to get the left side view of a binary tree?

easy

A. function leftSideView(root: number[]): number

B. function leftSideView(root: TreeNode | null): number[]

C. function leftSideView(root: TreeNode): void

D. function leftSideView(root: string): string[]

Solution

1. Step 1: Identify correct input type

   The function should accept a binary tree root node or null, so TreeNode | null is correct.

2. Step 2: Identify correct return type

   The function returns an array of numbers representing node values visible from the left side.

3. Final Answer:

   function leftSideView(root: TreeNode | null): number[] -> Option B

4. Quick Check:

   Input = TreeNode or null, Output = number array [OK]

Hint: Input is tree root or null; output is array of numbers [OK]

Common Mistakes:

• Using array of numbers as input instead of tree node
• Returning void instead of array
• Using wrong input type like string

3. Given the binary tree below, what is the left side view output?

    1
   / \
  2   3
   \   \
    4   5

Consider the TypeScript code:
console.log(leftSideView(root));

medium

A. [1, 2, 4]

B. [1, 3, 5]

C. [1, 2, 5]

D. [1, 4, 5]

Solution

1. Step 1: Identify nodes visible from left at each level

   Level 1: Node 1 is visible.
   Level 2: Node 2 is the leftmost node.
   Level 3: Node 4 is visible from left (child of 2).

2. Step 2: Collect visible nodes in order

   The left side view nodes are [1, 2, 4].

3. Final Answer:

   [1, 2, 4] -> Option A

4. Quick Check:

   Left side nodes = [1, 2, 4] [OK]

Hint: Pick first node at each level from left [OK]

Common Mistakes:

• Choosing right child nodes instead of left
• Including nodes not visible from left
• Confusing order of nodes

4. The following TypeScript code attempts to find the left side view of a binary tree but has a bug. What is the bug?

function leftSideView(root: TreeNode | null): number[] {
  const result: number[] = [];
  function dfs(node: TreeNode | null, level: number) {
    if (!node) return;
    if (result.length === level) {
      result.push(node.val);
    }
    dfs(node.right, level + 1);
    dfs(node.left, level + 1);
  }
  dfs(root, 0);
  return result;
}

medium

A. The result array is not initialized

B. The base case is missing to stop recursion

C. The function visits right child before left, so left side nodes are missed

D. The function does not return any value

Solution

1. Step 1: Analyze traversal order

   The code visits the right child before the left child, so it records right side nodes first.

2. Step 2: Understand impact on left side view

   Because right nodes are visited first, left side nodes at each level may be skipped, causing incorrect output.

3. Final Answer:

   The function visits right child before left, so left side nodes are missed -> Option C

4. Quick Check:

   Visit left child first for correct left side view [OK]

Hint: Traverse left child before right to capture left side nodes [OK]

Common Mistakes:

• Visiting right child before left child
• Forgetting base case (though present here)
• Not pushing node values at correct level

5. You want to find the left side view of a binary tree but only have the tree nodes stored in a nested object format without parent references. Which approach is best to implement in TypeScript to get the left side view efficiently?

hard

A. Use level order traversal (BFS) and pick the first node at each level

B. Use in-order traversal and collect nodes in an array

C. Use post-order traversal and reverse the result array

D. Use a stack to perform iterative right-to-left traversal

Solution

1. Step 1: Understand traversal types and their suitability

   Level order traversal (BFS) visits nodes level by level, making it easy to pick the first node at each level.

2. Step 2: Evaluate other traversal methods

   In-order and post-order traversals do not guarantee level-wise order. Right-to-left iterative traversal misses left side nodes.

3. Final Answer:

   Use level order traversal (BFS) and pick the first node at each level -> Option A

4. Quick Check:

   BFS level order = best for left side view [OK]

Hint: Use BFS and select first node per level for left side view [OK]

Common Mistakes:

• Using in-order or post-order which don't give level order
• Traversing right to left instead of left to right
• Ignoring level boundaries in traversal