Left Side View of Binary Tree in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Left Side View for a Simple Binary Tree

What is the output of the following TypeScript code that prints the left side view of a binary tree?

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    this.val = val === undefined ? 0 : val;
    this.left = left === undefined ? null : left;
    this.right = right === undefined ? null : right;
  }
}

function leftSideView(root: TreeNode | null): number[] {
  if (!root) return [];
  const result: number[] = [];
  const queue: (TreeNode | null)[] = [root];
  while (queue.length > 0) {
    const levelSize = queue.length;
    for (let i = 0; i < levelSize; i++) {
      const node = queue.shift()!;
      if (i === 0) result.push(node.val);
      if (node.left) queue.push(node.left);
      if (node.right) queue.push(node.right);
    }
  }
  return result;
}

const root = new TreeNode(1, new TreeNode(2, new TreeNode(4), null), new TreeNode(3));
console.log(leftSideView(root));

A [1, 2, 4]

B [1, 3, 4]

C [1, 2, 3]

D [1, 4]

💡 Hint

Remember, the left side view shows the first node visible at each level from the left side.

The left side view visits nodes level by level and picks the first node at each level. Here, level 1 is 1, level 2 is 2, and level 3 is 4.

🧠 Conceptual

intermediate

Understanding Left Side View Node Selection

In the left side view of a binary tree, which nodes are included in the output?

A The rightmost node at each level

B All leaf nodes

C The leftmost node at each level

D All nodes at the deepest level

💡 Hint

Think about what you see when looking at the tree from the left side.

The left side view includes the first node visible at each level when looking from the left side, which is the leftmost node at that level.

🔧 Debug

advanced

Identify the Bug in Left Side View Implementation

What error will the following TypeScript code produce when trying to get the left side view of a binary tree?

DSA Typescript

function leftSideView(root: TreeNode | null): number[] {
  if (!root) return [];
  const result: number[] = [];
  const queue: (TreeNode | null)[] = [root];
  while (queue.length > 0) {
    const levelSize = queue.length;
    for (let i = 0; i <= levelSize; i++) {
      const node = queue.shift()!;
      if (i === 0) result.push(node.val);
      if (node.left) queue.push(node.left);
      if (node.right) queue.push(node.right);
    }
  }
  return result;
}

A Runtime error: Cannot read property 'val' of undefined

B Syntax error: Unexpected token

C Output is missing some nodes but no error

D Correct output with no errors

💡 Hint

Check the loop condition carefully and how many times you remove nodes from the queue.

The for loop uses i <= levelSize, which causes one extra iteration. The queue.shift() returns undefined on that extra iteration, causing a runtime error when accessing node.val.

🚀 Application

advanced

Left Side View of an Unbalanced Binary Tree

Given the following unbalanced binary tree, what is the left side view output?

DSA Typescript

const root = new TreeNode(10,
  new TreeNode(5, null, new TreeNode(7, new TreeNode(6), null)),
  new TreeNode(15, null, new TreeNode(20))
);
console.log(leftSideView(root));

A [10, 15, 20]

B [10, 5, 6, 20]

C [10, 5, 7]

D [10, 5, 7, 6]

💡 Hint

Remember to pick the first node at each level from left to right.

Level 1: 10; Level 2: 5 (left child); Level 3: 7 (right child of 5, but no left child at this level); Level 4: 6 (left child of 7).

❓ Predict Output

expert

Output of Left Side View for Complex Tree with Nulls

What is the output of the following TypeScript code that prints the left side view of a binary tree with some null children?

DSA Typescript

const root = new TreeNode(1,
  new TreeNode(2, null, new TreeNode(5, new TreeNode(7), null)),
  new TreeNode(3, null, new TreeNode(4))
);
console.log(leftSideView(root));

A [1, 2, 7, 4]

B [1, 2, 5, 7]

C [1, 3, 4, 7]

D [1, 2, 5, 4]

💡 Hint

Trace each level and pick the first node encountered from left to right.

Level 1: 1; Level 2: 2 (left child); Level 3: 5 (right child of 2, no left child at this level); Level 4: 7 (left child of 5).

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