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Consider this modified code snippet for the Binary Tree Cameras problem. Which line contains the subtle bug that causes incorrect camera placement?

medium🐞 Bug Identification Q14 of Q15
Tree: Depth-First Search - Binary Tree Cameras
Consider this modified code snippet for the Binary Tree Cameras problem. Which line contains the subtle bug that causes incorrect camera placement?
ALine returning COVERED_NO_CAM after placing a camera instead of HAS_CAM
BLine returning NOT_COVERED at the end of dfs
CLine checking if dfs(root) == NOT_COVERED after traversal
DLine returning COVERED_NO_CAM when node is null
Step-by-Step Solution

  1. Step 1: Identify camera placement logic

    When a child is NOT_COVERED, a camera must be placed at current node and dfs must return HAS_CAM to indicate camera presence.

  2. Step 2: Locate incorrect return

    The code returns COVERED_NO_CAM after placing a camera, which falsely signals no camera here, causing parents to misinterpret coverage.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    Returning HAS_CAM is essential after placing a camera [OK]
Quick Trick: Return HAS_CAM after placing camera to signal coverage [OK]
Common Mistakes:
MISTAKES
  • Returning COVERED_NO_CAM instead of HAS_CAM after camera placement
  • Forgetting to add camera if root uncovered
  • Mixing coverage states in conditions
Trap Explanation:
PITFALL
  • Returning COVERED_NO_CAM looks plausible but breaks coverage propagation logic.
Interviewer Note:
CONTEXT
  • Checks candidate's attention to subtle state management bugs.
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