[Solved] Consider the following buggy code for checking if a binary tree is symmetric.... Which line contains the subtle bug that causes incorrect results for asymmetric trees? | Tree: Depth-First Search

Tree: Depth-First Search - Symmetric Tree (DFS Approach)

Consider the following buggy code for checking if a binary tree is symmetric. Which line contains the subtle bug that causes incorrect results for asymmetric trees?

def isSymmetric(root: Optional[TreeNode]) -> bool:
    def isMirror(t1: Optional[TreeNode], t2: Optional[TreeNode]) -> bool:
        if not t1 or not t2:
            return True
        if t1.val != t2.val:
            return False
        return isMirror(t1.left, t2.right) and isMirror(t1.right, t2.left)

    return isMirror(root.left, root.right) if root else True

ALine 5: if t1.val != t2.val: return False

BLine 3: if not t1 or not t2: return True

CLine 6: return isMirror(t1.left, t2.right) and isMirror(t1.right, t2.left)

DLine 8: return isMirror(root.left, root.right) if root else True

Step-by-Step Solution

Solution:

Step 1: Analyze base case for null nodes
The line returns true if either t1 or t2 is null, but it should only return true if both are null.
Step 2: Identify that returning true when only one node is null causes false positives
This causes asymmetric trees with missing nodes on one side to be incorrectly considered symmetric.
Final Answer:
Option B -> Option B
Quick Check:
Base case must check both nodes null equality, not just one [OK]

Quick Trick: Base case must return true only if both nodes are null [OK]

Common Mistakes:

MISTAKES

Returning true if only one node is null
Forgetting to check node values before recursion
Mixing left and right subtree comparisons

Trap Explanation:

PITFALL

Returning true on single null node looks plausible but breaks correctness on asymmetric trees.

Interviewer Note:

CONTEXT

Tests attention to base case correctness in recursive tree problems.

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More Tree: Depth-First Search Quizzes

Consider the following buggy code for checking if a binary tree is symmetric. Which line contains the subtle bug that causes incorrect results for asymmetric trees?

Step 1: Analyze base case for null nodes

Step 2: Identify that returning true when only one node is null causes false positives

Final Answer:

Quick Check:

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