Left Side View of Binary Tree in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a class for a binary tree node.

DSA Python

class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = [1]

Drag options to blanks, or click blank then click option'

Aval

B0

CNone

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or val instead of None for child nodes.

Assigning self to right child by mistake.

The left and right children of a node are initially set to None because they don't point to any node yet.

2fill in blank

medium

Complete the code to add the root node to the tree.

DSA Python

class BinaryTree:
    def __init__(self):
        self.root = [1]

Drag options to blanks, or click blank then click option'

ANone

B0

CNode()

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Creating a Node without value for root.

Assigning 0 or self to root.

The root starts as None because the tree is empty initially.

3fill in blank

hard

Fix the error in the function to get the left side view using level order traversal.

DSA Python

def left_side_view(root):
    if not root:
        return []
    queue = [root]
    result = []
    while queue:
        level_length = len(queue)
        for i in range(level_length):
            node = queue.pop(0)
            if i == [1]:
                result.append(node.val)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
    return result

Drag options to blanks, or click blank then click option'

A1

Blevel_length - 1

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using level_length - 1 which gives rightmost nodes.

Using i or 1 which do not select the leftmost node.

The left side view shows the first node at each level, which is when i == 0.

4fill in blank

hard

Fill both blanks to create a recursive helper function for left side view.

DSA Python

def left_view_helper(node, level, [1]):
    if not node:
        return
    if level == len([2]):
        result.append(node.val)
    left_view_helper(node.left, level + 1, result)
    left_view_helper(node.right, level + 1, result)

Drag options to blanks, or click blank then click option'

Aresult

Bqueue

Clevel

Dvisited

Attempts:

3 left

💡 Hint

Common Mistakes

Using queue or visited which are not used in recursion here.

Using level instead of result for length check.

The helper function uses 'result' to store left side nodes and checks its length to decide when to add a node.

5fill in blank

hard

Fill all three blanks to complete the recursive left side view function.

DSA Python

def left_side_view_recursive(root):
    result = []
    def helper(node, level, [1]):
        if not node:
            return
        if level == len([2]):
            [3].append(node.val)
        helper(node.left, level + 1, result)
        helper(node.right, level + 1, result)
    helper(root, 0, result)
    return result

Drag options to blanks, or click blank then click option'

Aresult

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the list.

Using queue which is not used in recursion.

The 'result' list is passed and used in all blanks to collect left side nodes during recursion.

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 Python code snippets correctly initializes an empty list to store the left side view nodes?

easy

A. left_view = ()

B. left_view = {}

C. left_view = []

D. left_view = set()

Solution

1. Step 1: Identify the correct data structure

   We need a list to store nodes in order, so an empty list is best.

2. Step 2: Check syntax for list initialization

   Using square brackets [] creates an empty list in Python.

3. Final Answer:

   left_view = [] -> Option C

4. Quick Check:

   Empty list = [] [OK]

Hint: Use [] to create empty list for ordered nodes [OK]

Common Mistakes:

• Using {} which creates a dictionary, not a list
• Using () which creates a tuple, not a list
• Using set() which is unordered

3. What is the output of the following code that prints the left side view of the given binary tree?

class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def left_side_view(root):
    result = []
    def dfs(node, level):
        if not node:
            return
        if level == len(result):
            result.append(node.val)
        dfs(node.left, level + 1)
        dfs(node.right, level + 1)
    dfs(root, 0)
    return result

root = Node(1)
root.left = Node(2)
root.right = Node(3)
root.left.right = Node(4)
root.right.right = Node(5)

print(left_side_view(root))

medium

A. [1, 2, 3]

B. [1, 2, 4]

C. [1, 3, 5]

D. [1, 4, 5]

Solution

1. Step 1: Trace DFS traversal and level checks

   At level 0, node 1 is added. At level 1, node 2 is added (left child). At level 2, node 4 is added (left child's right child).

2. Step 2: Confirm nodes added only once per level

   Right children at each level are ignored if left child exists at that level.

3. Final Answer:

   [1, 2, 4] -> Option B

4. Quick Check:

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

Hint: Add node if level equals result length during DFS [OK]

Common Mistakes:

• Adding right child nodes before left child nodes
• Appending nodes multiple times per level
• Confusing left and right children order

4. Find the error in the following code that attempts to print the left side view of a binary tree:

def left_side_view(root):
    result = []
    def dfs(node, level):
        if node is None:
            return
        if level < len(result):
            result.append(node.val)
        dfs(node.left, level + 1)
        dfs(node.right, level + 1)
    dfs(root, 0)
    return result

medium

A. The condition should be 'level == len(result)', not 'level < len(result)'

B. The base case should check 'if node' instead of 'if node is None'

C. The recursive calls should swap left and right children

D. The function should return 'result' inside the dfs function

Solution

1. Step 1: Analyze the condition for adding nodes

   The code uses 'level < len(result)' which is incorrect because nodes should be added only when level equals the current result length.

2. Step 2: Correct the condition to add nodes once per level

   Changing condition to 'level == len(result)' ensures only the first node at each level is added.

3. Final Answer:

   The condition should be 'level == len(result)', not 'level < len(result)' -> Option A

4. Quick Check:

   Use '==' to add first node per level [OK]

Hint: Add node only when level equals result length [OK]

Common Mistakes:

• Using '<' instead of '==' in condition
• Returning result inside recursive function
• Swapping left and right child calls unnecessarily

5. Given a binary tree, how would you modify the standard left side view algorithm to return the left side view nodes in reverse order (bottom to top)?

hard

A. Perform the usual left side view traversal, then reverse the result list before returning

B. Traverse the tree from right to left instead of left to right

C. Add nodes to the result list only if they are leaf nodes

D. Use a queue and perform level order traversal from right to left

Solution

1. Step 1: Perform standard left side view traversal

   Use DFS or BFS to collect the first node at each level from left to right.

2. Step 2: Reverse the collected list

   After traversal, reverse the list to get nodes from bottom to top.

3. Final Answer:

   Perform the usual left side view traversal, then reverse the result list before returning -> Option A

4. Quick Check:

   Reverse after left side view traversal [OK]

Hint: Reverse the left side view list after traversal [OK]

Common Mistakes:

• Changing traversal order instead of reversing result
• Collecting leaf nodes only
• Using right to left traversal which gives wrong nodes