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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of XOR to find unique element

What is the output of the following Java code that uses XOR to find the only non-repeating element in the array?

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {2, 3, 5, 4, 5, 3, 4};
        int result = 0;
        for (int num : arr) {
            result ^= num;
        }
        System.out.println(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.

The XOR operation cancels out all numbers appearing twice, leaving only the unique number. Here, 2 appears once, so the output is 2.

❓ Predict Output

intermediate

Result after XOR operations

What will be printed after running this Java code snippet?

DSA Java

int[] nums = {7, 9, 7, 9, 11};
int unique = 0;
for (int n : nums) {
    unique ^= n;
}
System.out.println(unique);

A 0

B 7

C 9

D 11

💡 Hint

Pairs cancel out with XOR, leaving the unique number.

7 and 9 appear twice and cancel out. 11 appears once, so the output is 11.

🔧 Debug

advanced

Identify the error in XOR unique element code

What error will this Java code produce when trying to find the unique element using XOR?

DSA Java

int[] arr = {1, 2, 1, 3, 2};
int result = 0;
for (int num : arr) {
    result ^= num;
}
System.out.println(result);

A Compilation error: variable 'result' might not have been initialized

B Output: 3

C Runtime error: NullPointerException

D Compilation error: missing semicolon

💡 Hint

Check if all variables are initialized before use.

The variable 'result' was not initialized before XOR operation, causing a compilation error. Initializing it to 0 fixes the error and outputs 3.

🧠 Conceptual

advanced

Why XOR works for finding the unique element?

Why does XOR operation help in finding the only non-repeating element in an array where every other element repeats twice?

A Because XOR of a number with itself is zero and XOR with zero is the number itself

B Because XOR adds all numbers together

C Because XOR multiplies all numbers together

D Because XOR sorts the array automatically

💡 Hint

Think about how pairs cancel out using XOR.

XOR cancels out pairs because n ^ n = 0, and 0 ^ n = n, leaving only the unique element.

❓ Predict Output

expert

Output of XOR with multiple unique elements

What will be the output of this Java code that uses XOR on an array with two unique elements?

DSA Java

int[] arr = {4, 1, 2, 1, 2, 5};
int xorResult = 0;
for (int num : arr) {
    xorResult ^= num;
}
System.out.println(xorResult);

A 2

B 0

C 1

D 1_alt

💡 Hint

XOR of two different unique numbers is not zero, check carefully.

The array has two unique numbers 4 and 5. XOR of 4 and 5 is 1, and since 1 appears twice, it cancels out, leaving 4 ^ 5 = 1. So output is 1.

Practice

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

easy

A. It returns 0

B. It returns the number itself

C. It returns 1

D. It returns -1

Solution

1. Step 1: Understand XOR property for identical numbers

   XOR of a number with itself always results in 0 because bits cancel out.

2. Step 2: Apply XOR on example

   For example, 5 XOR 5 = 0.

3. Final Answer:

   It returns 0 -> Option A

4. Quick Check:

   X XOR X = 0 [OK]

Hint: X XOR X always equals zero [OK]

Common Mistakes:

• Thinking XOR returns the number itself
• Confusing XOR with OR
• Assuming XOR returns 1 for identical numbers

2. Which of the following is the correct syntax to initialize an integer variable named result to 0 in Java?

easy

A. int result := 0;

B. int result == 0;

C. int result = 0;

D. int result = zero;

Solution

1. Step 1: Recall Java variable initialization syntax

   In Java, variables are declared with type, name, and assignment using single equals sign.

2. Step 2: Check each option

   int result = 0; uses correct syntax: int result = 0;. Others have syntax errors.

3. Final Answer:

   int result = 0; -> Option C

4. Quick Check:

   Correct Java variable initialization [OK]

Hint: Use single '=' for assignment in Java [OK]

Common Mistakes:

• Using '==' instead of '='
• Using ':=' which is not Java syntax
• Assigning undefined variable 'zero'

3. What will be the output of the following Java code?

int[] arr = {2, 3, 2, 4, 3};
int result = 0;
for (int num : arr) {
    result ^= num;
}
System.out.println(result);

medium

A. 4

B. 3

C. 2

D. 0

Solution

1. Step 1: Apply XOR operation on array elements

   Start with result=0, XOR with each element: 0^2=2, 2^3=1, 1^2=3, 3^4=7, 7^3=4.

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

   The only element that does not repeat is 4, which is the final XOR result.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Only non-repeating element = 4 [OK]

Hint: XOR all elements; result is unique number [OK]

Common Mistakes:

• Not starting XOR with 0
• Miscomputing XOR steps
• Assuming sum instead of XOR

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

int[] arr = {1, 2, 1, 3, 2};
int result;
for (int num : arr) {
    result ^= num;
}
System.out.println(result);

medium

A. System.out.println syntax is wrong

B. Array 'arr' is not declared properly

C. For-each loop syntax is incorrect

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

Solution

1. Step 1: Check variable initialization

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

2. Step 2: Correct initialization needed

   Initialize 'result' to 0 before the loop to avoid error.

3. Final Answer:

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

4. Quick Check:

   Uninitialized variable causes error [OK]

Hint: Always initialize XOR accumulator to 0 [OK]

Common Mistakes:

• Forgetting to initialize result
• Misusing loop syntax
• Incorrect print statement

5. Given an array where every element repeats twice except one element which repeats thrice, can the XOR method find the unique element? What will be the output of this code?

int[] arr = {5, 5, 5, 7, 7, 8, 8};
int result = 0;
for (int num : arr) {
    result ^= num;
}
System.out.println(result);

hard

A. 5

B. Cannot find unique element correctly

C. 0

D. 7

Solution

1. Step 1: Understand XOR property for odd/even counts

   XOR cancels even number of identical elements to 0; odd count leaves the element.

2. Step 2: Analyze given array and XOR result

   5 appears 3 times (odd) -> XOR of three identical numbers is not the number itself; XOR of 5^5=0, 0^5=5, but this pattern does not hold for triple repeats in general.

3. Step 3: Conclusion

   XOR method cannot reliably find the unique element when counts are not exactly twice except one.

4. Final Answer:

   Cannot find unique element correctly -> Option B

5. Quick Check:

   XOR fails for elements repeating thrice [OK]

Hint: XOR only works when all elements except one appear exactly twice [OK]

Common Mistakes:

• Thinking XOR finds unique element with triple repeats
• Miscomputing XOR of three identical numbers as the number itself
• Ignoring that XOR only cancels pairs