Find the Only Non Repeating Element Using XOR in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the result variable to zero.

DSA C#

int result = [1];

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

A1

Bnull

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing result with 1 or -1 causes wrong output.

Using null is invalid for int type.

We start with zero because XOR with zero leaves the number unchanged.

2fill in blank

medium

Complete the code to XOR each element in the array with the result.

DSA C#

foreach (int num in arr) {
    result = result [1] num;
}

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

A^

B+

C&

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or & instead of ^ changes the logic.

Using | will not cancel duplicates.

XOR operator (^) is used to find the unique element by cancelling out duplicates.

3fill in blank

hard

Fix the error in the return statement to return the unique element.

DSA C#

return [1];

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

Aresult

Bnum

Carr[0]

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning arr[0] assumes first element is unique.

Returning 0 ignores the computed result.

The variable 'result' holds the unique non-repeating element after XOR operations.

4fill in blank

hard

Fill both blanks to complete the method signature and the loop over the array.

DSA C#

public static int FindUnique([1] arr) {
    int result = 0;
    foreach ([2] num in arr) {
        result ^= num;
    }
    return result;
}

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

Aint[]

BList<int>

Cint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of int[] for the parameter.

Using List but iterating with int is okay but inconsistent here.

Using int for the parameter instead of a collection.

The method takes an array of integers (int[]) and iterates over each integer (int) in the array.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that counts occurrences and filters unique elements.

DSA C#

var counts = new Dictionary<int, int>();
foreach (var num in arr) {
    if (!counts.ContainsKey([1])) {
        counts[[2]] = 0;
    }
    counts[num] += 1;
}
var unique = counts.Where(kv => kv.Value == [3]).Select(kv => kv.Key).ToList();

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

Anum

B1

C0

Dcounts

Attempts:

3 left

💡 Hint

Common Mistakes

Using counts instead of num as key.

Initializing count to 1 instead of 0.

Filtering for value == 0 instead of 1.

We check if 'num' is in counts, initialize counts[num] to 0, then count occurrences. Unique elements have count 1 after initialization and increment.

Practice

1. What does the XOR operation do when applied between two identical numbers?

easy

A. It returns the number itself

B. It causes an error

C. It returns 1

D. It returns 0

Solution

1. Step 1: Understand XOR behavior on identical bits

   XOR returns 0 when both bits are the same (0 XOR 0 = 0, 1 XOR 1 = 0).

2. Step 2: Apply XOR on identical numbers

   Since all bits are identical, XOR of a number with itself results in 0.

3. Final Answer:

   It returns 0 -> Option D

4. Quick Check:

   5 XOR 5 = 0 [OK]

Hint: XOR of same numbers always zeroes out [OK]

Common Mistakes:

• Thinking XOR returns the number itself
• Confusing XOR with OR or AND
• Assuming XOR causes errors with identical inputs

2. Which of the following is the correct syntax to initialize an integer variable named result to zero in C#?

easy

A. int result => 0;

B. int result == 0;

C. int result = 0;

D. int result := 0;

Solution

1. Step 1: Recall C# variable initialization syntax

   In C#, variables are initialized using single equals sign (=) for assignment.

2. Step 2: Check each option

   int result = 0; uses correct syntax: int result = 0;. Others use invalid operators.

3. Final Answer:

   int result = 0; -> Option C

4. Quick Check:

   Correct assignment uses = in C# [OK]

Hint: Use single = for assignment in C# [OK]

Common Mistakes:

• Using double equals (==) for assignment
• Using := or => which are invalid here
• Missing semicolon at the end

3. What is the output of the following C# code?

int[] arr = {2, 3, 2, 4, 3};
int result = 0;
foreach (int num in arr) {
    result ^= num;
}
Console.WriteLine(result);

medium

A. 4

B. 3

C. 2

D. 0

Solution

1. Step 1: XOR all elements in the array

   Start with result = 0. XOR with 2 -> 0^2=2, then 3 -> 2^3=1, then 2 -> 1^2=3, then 4 -> 3^4=7, then 3 -> 7^3=4.

2. Step 2: Final XOR result is the non-repeating element

   The final result after XORing all elements is 4, which is the only non-repeating number.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Only 4 appears once, XOR result = 4 [OK]

Hint: XOR all numbers; duplicates cancel out [OK]

Common Mistakes:

• Not XORing all elements correctly
• Confusing XOR with addition
• Assuming first unique number is output

4. Identify the error in the following C# code that tries to find the only non-repeating element using XOR:

int[] arr = {1, 2, 1, 3, 2};
int result;
foreach (int num in arr) {
    result ^= num;
}
Console.WriteLine(result);

medium

A. Variable 'result' is not initialized before XOR operation

B. Using '^=' operator is incorrect for XOR in C#

C. Array syntax is invalid

D. Console.WriteLine cannot print integers

Solution

1. Step 1: Check variable initialization

   The variable 'result' is declared but not initialized before using '^=' operator, which causes a compile-time error.

2. Step 2: Correct initialization needed

   Initialize 'result' to 0 before the loop to correctly accumulate XOR results.

3. Final Answer:

   Variable 'result' is not initialized before XOR operation -> Option A

4. Quick Check:

   Uninitialized variables cause errors in C# [OK]

Hint: Always initialize variables before use [OK]

Common Mistakes:

• Forgetting to initialize variables
• Misunderstanding '^=' operator usage
• Assuming Console.WriteLine can't print ints

5. Given an array where every element appears twice except one element which appears once, how would you modify the XOR approach to find the unique element if the array is very large and you want to optimize memory usage?

hard

A. Sort the array first then scan for unique element

B. Use XOR in a single pass without extra memory

C. Use a dictionary to count occurrences

D. Copy array to a new list and remove duplicates

Solution

1. Step 1: Understand XOR single pass advantage

   XOR operation can be done in one pass over the array without extra memory, as duplicates cancel out.

2. Step 2: Avoid extra memory by not using sorting or dictionaries

   Sorting or dictionaries use extra memory or time, which is inefficient for large arrays.

3. Final Answer:

   Use XOR in a single pass without extra memory -> Option B

4. Quick Check:

   XOR single pass = memory efficient [OK]

Hint: XOR once over array; no extra memory needed [OK]

Common Mistakes:

• Using extra data structures unnecessarily
• Sorting which costs extra time and memory
• Copying arrays wastes memory