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The following code attempts to find two missing numbers using XOR partitioning but sometimes returns incorrect results. Identify the bug.

medium🐞 Bug Identification Q14 of Q15
Bit Manipulation - Find Two Missing Numbers
The following code attempts to find two missing numbers using XOR partitioning but sometimes returns incorrect results. Identify the bug.
AFinal return statement returning [x, y]
BLine calculating xor_all with XOR of all numbers
CLoop partitioning numbers based on set_bit
DLine calculating set_bit using bitwise OR instead of AND
Step-by-Step Solution

  1. Step 1: Understand set_bit calculation

    Correct approach finds rightmost set bit using xor_all & (-xor_all). Using OR instead breaks partitioning.

  2. Step 2: Consequence of bug

    Incorrect set_bit causes wrong grouping of numbers, leading to incorrect missing numbers returned.

  3. Final Answer:

    Option D -> Option D

  4. Quick Check:

    Replacing OR with AND fixes partitioning and output [OK]
Quick Trick: Rightmost set bit must be found with AND, not OR [OK]
Common Mistakes:
MISTAKES
  • Using OR instead of AND for set bit
  • Misplacing XOR operations
  • Returning wrong variables
Trap Explanation:
PITFALL
  • OR looks similar to AND but changes bit pattern drastically, causing subtle bugs.
Interviewer Note:
CONTEXT
  • Tests candidate's attention to subtle bitwise operation details in code.
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