Find the Only Non Repeating Element Using XOR in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code that finds the only non-repeating element using XOR?

Consider the following C# code that uses XOR to find the unique element in an array where every other element repeats twice. What will be printed?

DSA C#

using System;
class Program {
    static void Main() {
        int[] arr = {2, 3, 5, 4, 5, 3, 4};
        int result = 0;
        foreach (int num in arr) {
            result ^= num;
        }
        Console.WriteLine(result);
    }
}

A 2

B 5

C 3

D 4

💡 Hint

XOR of a number with itself is 0, and XOR with 0 is the number itself.

All numbers except one appear twice. XOR cancels out pairs, leaving the unique number.

❓ Predict Output

intermediate

What is the output of this XOR-based unique element finder with negative numbers?

What will this C# program print when the array contains negative and positive integers with one unique element?

DSA C#

using System;
class Program {
    static void Main() {
        int[] arr = {-1, -2, -2, -1, -3};
        int unique = 0;
        foreach (int n in arr) {
            unique ^= n;
        }
        Console.WriteLine(unique);
    }
}

A -3

B -2

C -1

D 0

💡 Hint

XOR works the same with negative numbers as with positive.

Pairs cancel out, leaving the unique number -3.

🧠 Conceptual

advanced

Why does XOR help find the only non-repeating element in an array?

Which of the following best explains why XOR operation helps find the unique element in an array where every other element repeats twice?

A XOR multiplies all numbers, so the unique number is the product divided by duplicates.

B XOR adds all numbers together, so the unique number is the sum minus duplicates.

C XOR of a number with itself is zero, so pairs cancel out, leaving the unique number.

D XOR sorts the array, making it easy to find the unique element.

💡 Hint

Think about what happens when you XOR a number twice.

XOR of a number with itself is zero, so pairs cancel out, leaving only the unique number.

❓ Predict Output

advanced

What is the output of this code with multiple unique elements?

What will this C# program print if the array contains two unique elements and others repeated twice?

DSA C#

using System;
class Program {
    static void Main() {
        int[] arr = {1, 2, 3, 2, 1, 4};
        int result = 0;
        foreach (int num in arr) {
            result ^= num;
        }
        Console.WriteLine(result);
    }
}

A 4

B 0

C 3

D 7

💡 Hint

XOR of two unique numbers is their XOR value, not one of them.

XOR of 3 and 4 is 7, so the output is 7.

🔧 Debug

expert

What error does this code produce when finding the unique element using XOR?

This C# code attempts to find the unique element using XOR but causes an error. What error will it produce?

DSA C#

using System;
class Program {
    static void Main() {
        int[] arr = {1, 2, 1, 3, 2};
        int result;
        foreach (int num in arr) {
            result ^= num;
        }
        Console.WriteLine(result);
    }
}

A Compile-time error: Missing semicolon

B Compile-time error: Use of unassigned local variable 'result'

C Output: 3

D Runtime error: NullReferenceException

💡 Hint

Check if 'result' is initialized before use.

Variable 'result' is declared but not initialized before XOR operation, causing a compile error.

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