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In the following Python code to find the missing number using XOR, which line causes incorrect results when the missing number is n?

medium🔍🐞 Bug Finding Q7 of Q15

Bit Manipulation - Find the Missing Number

In the following Python code to find the missing number using XOR, which line causes incorrect results when the missing number is n?

def find_missing(nums):
    n = len(nums)
    xor_val = 0
    for i in range(n):
        xor_val ^= nums[i]
    for i in range(n):
        xor_val ^= i
    return xor_val

ALine 6: for i in range(n): xor_val ^= i

BLine 4: for i in range(n): xor_val ^= nums[i]

CLine 3: xor_val = 0

DLine 2: n = len(nums)

Step-by-Step Solution

Solution:

Step 1: Identify the range of numbers
The array contains numbers from 0 to n, but the second loop only XORs numbers from 0 to n-1.
Step 2: Spot the bug
Line 6 loops from 0 to n-1, missing the number n, so if n is missing, it won't be XORed back.
Final Answer:
Option A -> Option A
Quick Check:
Range should be 0 to n inclusive [OK]

Quick Trick: XOR all numbers including n, not just up to n-1 [OK]

Common Mistakes:

MISTAKES

Assuming range(n) includes n
Not considering missing number could be n
Ignoring off-by-one errors in loops

Trap Explanation:

PITFALL

Range excludes n, so missing number n is never XORed back.

Interviewer Note:

CONTEXT

Tests ability to detect off-by-one bugs in bitwise algorithms.

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