Left Side View of Binary Tree in DSA C# - 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 technique 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 C#, what data structure is typically used to implement Level Order Traversal for the Left Side View?

A Queue is typically used to hold nodes of each level during traversal. It helps process nodes in FIFO order, level by level.

beginner

Why do we pick the first node at each level during Level Order Traversal for the Left Side View?

The first node at each level is the leftmost node visible from the left side. Other nodes at the same level are hidden behind it.

intermediate

What is the time complexity of finding the Left Side View of a Binary Tree using Level Order Traversal?

The time complexity is O(n), where n is the number of nodes in the tree, because each node is visited exactly once.

What data structure is best suited for Level Order Traversal in C#?

A List

B Stack

C Dictionary

D Queue

Queue is used because it processes nodes in FIFO order, which is needed for level by level traversal.

Which node do we select at each level to form the Left Side View?

A Rightmost node

B Last node visited

C Leftmost node

D Any node at that level

The leftmost node at each level is visible from the left side and forms the Left Side View.

What is the time complexity of the Left Side View algorithm using Level Order Traversal?

A O(log n)

B O(n)

C O(n^2)

D O(1)

Each node is visited once, so the time complexity is O(n).

If a binary tree has 3 levels, how many nodes will the Left Side View contain at most?

A 3

B 2

C 1

D Depends on the number of nodes

At most one node per level is visible from the left side, so for 3 levels, up to 3 nodes.

Which traversal order is NOT suitable for finding the Left Side View?

A Preorder Traversal

B Queue-based traversal

C Breadth-First Search

D Level Order Traversal

Preorder is depth-first and does not guarantee visiting nodes level by level, so it is not suitable.

Explain how to find the Left Side View of a Binary Tree using Level Order Traversal.

Think about visiting nodes one level at a time and picking the leftmost node.

Describe the time complexity and why it is that for the Left Side View algorithm.

Consider how many times each node is processed.

Practice

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

easy

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

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

C. All leaf nodes of the tree

D. Nodes at the deepest level only

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 A

4. Quick Check:

   Left side view = nodes seen from left [OK]

Hint: Think of standing on the left side and looking at the tree [OK]

Common Mistakes:

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

2. Which C# data structure is best suited to help find the left side view of a binary tree level by level?

easy

A. Stack

B. Dictionary

C. Queue

D. List

Solution

1. Step 1: Identify traversal method

   Left side view is found by level order traversal, which processes nodes level by level.

2. Step 2: Choose data structure for level order

   A queue is used to hold nodes of each level in order for processing.

3. Final Answer:

   Queue -> Option C

4. Quick Check:

   Level order traversal uses queue [OK]

Hint: Use queue for level-by-level tree traversal [OK]

Common Mistakes:

• Using stack which is for depth-first traversal
• Choosing dictionary which stores key-value pairs
• Using list without order control

3. What is the output of the left side view for this binary tree?

    1
   / \
  2   3
 /     \
4       5

medium

A. [1, 2, 4]

B. [1, 3, 5]

C. [4, 2, 1]

D. [1, 2, 5]

Solution

1. Step 1: Identify nodes visible from left side

   At level 1: node 1 is visible; level 2: node 2; level 3: node 4.

2. Step 2: List nodes in order of levels

   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 side nodes instead
• Including nodes not visible from left
• Reversing order of nodes

4. Consider this C# code snippet to get the left side view of a binary tree:

public IList<int> LeftSideView(TreeNode root) {
    var result = new List<int>();
    if (root == null) return result;
    var queue = new Queue<TreeNode>();
    queue.Enqueue(root);
    while (queue.Count > 0) {
        int size = queue.Count;
        for (int i = 0; i < size; i++) {
            var node = queue.Dequeue();
            if (i == 0) result.Add(node.val);
            if (node.left != null) queue.Enqueue(node.left);
            if (node.right != null) queue.Enqueue(node.right);
        }
    }
    return result;
}

What is the bug in this code?

medium

A. It misses the root node in the result

B. It uses a stack instead of a queue

C. It adds the rightmost node instead of leftmost

D. It correctly returns the left side view

Solution

1. Step 1: Analyze the traversal logic

   The code uses a queue for level order traversal and adds the first node at each level to result.

2. Step 2: Verify correctness of node selection

   Adding node when i == 0 ensures leftmost node at each level is added.

3. Final Answer:

   It correctly returns the left side view -> Option D

4. Quick Check:

   First node per level added = left side view [OK]

Hint: Check if first node at each level is added [OK]

Common Mistakes:

• Confusing queue with stack usage
• Adding last node instead of first
• Skipping root node

5. You want to find the left side view of a binary tree but only have access to a recursive preorder traversal method. How can you modify the recursion to get the left side view?

hard

A. Traverse left child before right, but record only leaf nodes

B. Traverse left child before right, and record the first node visited at each depth level

C. Traverse right child before left, and record all nodes at each level

D. Traverse right child before left, and record the last node visited at each depth level

Solution

1. Step 1: Understand preorder traversal order

   Preorder visits root, then left, then right child recursively.

2. Step 2: Record first node at each depth

   By visiting left child first, the first node at each depth is the leftmost node.

3. Step 3: Skip recording if depth already visited

   Only add node value if this is the first visit at that depth.

4. Final Answer:

   Traverse left child before right, and record the first node visited at each depth level -> Option B

5. Quick Check:

   Left-first preorder + first node per depth = left side view [OK]

Hint: Visit left child first and record first node at each depth [OK]

Common Mistakes:

• Visiting right child first loses leftmost nodes
• Recording all nodes instead of first per level
• Recording only leaf nodes misses intermediate nodes