Mirror a Binary Tree in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap the left and right children of the root node.

DSA Python

def mirror(root):
    if root is None:
        return
    root.left, root.[1] = root.[1], root.left

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

Aparent

Bleft

Cright

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with 'left' instead of 'right' causes no change.

Trying to swap with 'parent' or 'child' which are not valid attributes.

Swapping root.left and root.right mirrors the root node's children.

2fill in blank

medium

Complete the code to recursively mirror the left subtree.

DSA Python

def mirror(root):
    if root is None:
        return
    root.left, root.right = root.right, root.left
    mirror(root.[1])

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

Aparent

Bchild

Cright

Dleft

Attempts:

3 left

💡 Hint

Common Mistakes

Calling mirror on root.right only misses the left subtree.

Using 'parent' or 'child' which are not valid attributes.

We call mirror on root.left to recursively mirror the left subtree.

3fill in blank

hard

Fix the error in the recursive call to mirror the right subtree.

DSA Python

def mirror(root):
    if root is None:
        return
    root.left, root.right = root.right, root.left
    mirror(root.left)
    mirror(root.[1])

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

Aparent

Bright

Cleft

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Calling mirror on root.left twice.

Using invalid attributes like 'parent' or 'child'.

We must call mirror on root.right to mirror the right subtree.

4fill in blank

hard

Fill both blanks to complete the mirror function that swaps children and recursively mirrors subtrees.

DSA Python

def mirror(root):
    if root is None:
        return
    root.[1], root.[2] = root.[2], root.[1]
    mirror(root.left)
    mirror(root.right)

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

Aleft

Bright

Cparent

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with 'parent' or 'child' which are invalid.

Swapping the same attribute with itself.

Swapping root.left and root.right mirrors the node's children correctly.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps node values to their mirrored subtree sizes if size > 0.

DSA Python

def subtree_size(node):
    if node is None:
        return 0
    return 1 + subtree_size(node.left) + subtree_size(node.right)

sizes = {node.[1]: subtree_size(node) for node in nodes if subtree_size(node) [2] 0 and node.[3] is not None}

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

Aval

B>

Cleft

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'val' incorrectly or missing the condition.

Using wrong comparison operators.

Checking the wrong child attribute.

We map node.val to subtree_size(node) if subtree_size(node) > 0 and node.left is not None.

Practice

1. What does mirroring a binary tree mean?
Mirror a Binary Tree means:

easy

A. Swapping the left and right children of every node

B. Deleting all leaf nodes

C. Reversing the values of nodes in each level

D. Sorting the nodes in ascending order

Solution

1. Step 1: Understand the mirror operation

   Mirroring a binary tree means swapping the left and right children of every node in the tree.

2. Step 2: Compare with other options

   Deleting leaves, reversing values, or sorting nodes are different operations and do not create a mirror image.

3. Final Answer:

   Swapping the left and right children of every node -> Option A

4. Quick Check:

   Mirror = Swap children [OK]

Hint: Mirror means swap left and right children at all nodes [OK]

Common Mistakes:

• Thinking mirror means reversing node values only
• Confusing mirror with tree traversal
• Assuming mirror deletes nodes

2. Which of the following is the correct Python function header to define a recursive mirror function for a binary tree node named root?

easy

A. def mirror(root):

B. def mirror(root) -> int:

C. def mirror(root) -> None:

D. def mirror(root) -> str:

Solution

1. Step 1: Identify function purpose and return type

   The mirror function modifies the tree in place and returns nothing, so return type is None.

2. Step 2: Check syntax correctness

   def mirror(root) -> None: uses correct Python syntax with type hint for no return value.

3. Final Answer:

   def mirror(root) -> None: -> Option C

4. Quick Check:

   Function returns nothing = None [OK]

Hint: Mirror modifies tree, returns nothing, so use -> None [OK]

Common Mistakes:

• Using wrong return type like int or str
• Omitting parentheses in function definition
• Missing colon at end of function header

3. What will be the output of the following code after calling mirror(root)? Assume the tree is:
1
/ \
2 3

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

def mirror(root) -> None:
    if root is None:
        return
    root.left, root.right = root.right, root.left
    mirror(root.left)
    mirror(root.right)

root = Node(1)
root.left = Node(2)
root.right = Node(3)
mirror(root)
print(root.left.val, root.right.val)

medium

A. 1 3

B. 3 2

C. 2 3

D. 3 1

Solution

1. Step 1: Understand initial tree structure

   Root is 1, left child 2, right child 3.

2. Step 2: Apply mirror function

   Mirror swaps left and right children at root, so left becomes 3 and right becomes 2.

3. Final Answer:

   3 2 -> Option B

4. Quick Check:

   Swapped children = 3 2 [OK]

Hint: Mirror swaps children, so left=3 and right=2 after mirror [OK]

Common Mistakes:

• Not swapping children correctly
• Printing values before mirror call
• Confusing left and right child values

4. Find the error in this mirror function code:

def mirror(root):
    if root is None:
        return
    mirror(root.left)
    mirror(root.right)
    root.left = root.right
    root.right = root.left

medium

A. Swapping left and right children is done incorrectly

B. Base case is missing

C. Function does not call itself recursively

D. No return statement in function

Solution

1. Step 1: Analyze swapping logic

   Assigning root.left = root.right then root.right = root.left sets both to same child, losing original left.

2. Step 2: Correct swapping method

   Use simultaneous swap: root.left, root.right = root.right, root.left to avoid overwriting.

3. Final Answer:

   Swapping left and right children is done incorrectly -> Option A

4. Quick Check:

   Swap must be simultaneous to avoid overwrite [OK]

Hint: Swap children simultaneously to avoid overwriting [OK]

Common Mistakes:

• Swapping children one by one causing data loss
• Forgetting base case for recursion
• Not swapping children at all

5. Given a binary tree, write a function to check if the tree is symmetric (a mirror of itself). Which approach correctly uses the mirror concept?

hard

A. Traverse tree in preorder and compare with inorder traversal

B. Check if left and right subtrees have the same number of nodes

C. Check if all node values are equal in left and right subtrees

D. Compare left subtree with right subtree by recursively checking if left.left matches right.right and left.right matches right.left

Solution

1. Step 1: Understand symmetry in binary trees

   A symmetric tree is a mirror of itself, so left subtree must mirror right subtree.

2. Step 2: Use recursive comparison

   Check if left.left matches right.right and left.right matches right.left recursively to confirm mirror symmetry.

3. Final Answer:

   Compare left subtree with right subtree by recursively checking if left.left matches right.right and left.right matches right.left -> Option D

4. Quick Check:

   Symmetry = recursive mirror check [OK]

Hint: Symmetry means left subtree mirrors right subtree recursively [OK]

Common Mistakes:

• Only comparing node counts, not structure
• Comparing node values without structure
• Using traversal orders that don't check mirror