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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the result variable to zero.

DSA Java

int result = [1];

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A0

B-1

Cnull

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing result with 1 or -1 causes incorrect XOR results.

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 for-loop header to iterate over the array named nums.

DSA Java

for (int [1] = 0; i < nums.length; i++) {

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Aj

Bi

Cindex

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable different from i causes compilation errors.

Using variables not declared in the loop header.

The variable i is commonly used as the loop counter in Java.

3fill in blank

hard

Fix the error in the XOR operation inside the loop.

DSA Java

result = result [1] nums[i];

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A+

B-

C^

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' or '-' instead of '^' changes the logic.

Using '&' performs bitwise AND, not XOR.

XOR operator in Java is '^'. It is used to find the non-repeating element.

4fill in blank

hard

Fill both blanks to complete the method signature and return statement.

DSA Java

public static int [1](int[] nums) {
    int result = 0;
    for (int i = 0; i < nums.length; i++) {
        result = result ^ nums[i];
    }
    return [2];
}

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

AfindUnique

BuniqueNumber

Cresult

Dnums

Attempts:

3 left

💡 Hint

Common Mistakes

Returning nums instead of result.

Using incorrect method names that don't match usage.

The method is named findUnique and returns the variable result which holds the unique element.

5fill in blank

hard

Fill all three blanks to complete the main method that tests the unique element finder.

DSA Java

public static void main(String[] args) {
    int[] nums = {2, 3, 5, 4, 5, 3, 4};
    int unique = [1](nums);
    System.out.println("Unique element is: " + [2]);
    // Expected output: Unique element is: [3]
}

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

AfindUnique

Bunique

C2

Dnums

Attempts:

3 left

💡 Hint

Common Mistakes

Printing the array instead of the unique element.

Calling a method name that doesn't exist.

The method findUnique is called with nums array, result stored in unique, and printed. The unique element is 2.

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