Two Non Repeating Elements in Array Using XOR in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of XOR-based two unique elements finder

What is the output of the following Python code that finds two non-repeating elements in an array using XOR?

DSA Python

def find_two_unique(arr):
    xor_all = 0
    for num in arr:
        xor_all ^= num
    set_bit = xor_all & (-xor_all)
    num1 = 0
    num2 = 0
    for num in arr:
        if num & set_bit:
            num1 ^= num
        else:
            num2 ^= num
    return num1, num2

result = find_two_unique([2, 3, 7, 9, 2, 3])
print(result)

A (7, 9)

B (9, 7)

C (2, 3)

D (0, 0)

💡 Hint

Remember XOR of two same numbers is zero and XOR of all elements gives XOR of unique elements.

The code uses XOR to find the XOR of the two unique numbers (7 and 9). Then it finds a set bit to separate numbers into two groups and XORs them separately to get the unique numbers. The order returned is (7, 9).

❓ Predict Output

intermediate

Output of XOR partitioning step

What is the value of 'set_bit' after executing this code snippet?

DSA Python

arr = [4, 1, 2, 1, 2, 5]
xor_all = 0
for num in arr:
    xor_all ^= num
set_bit = xor_all & (-xor_all)
print(set_bit)

A 0

B 4

C 5

D 1

💡 Hint

The set_bit isolates the rightmost set bit of xor_all.

xor_all is XOR of all elements: 4^1^2^1^2^5 = 4^0^0^5 = 1 (since pairs cancel out). The rightmost set bit of 1 is 1 itself.

🔧 Debug

advanced

Identify the error in XOR two unique elements code

What error will this code produce when run?

DSA Python

def find_two_unique(arr):
    xor_all = 0
    for num in arr:
        xor_all ^= num
    set_bit = xor_all & (-xor_all)
    num1 = 0
    num2 = 0
    for num in arr:
        if num & set_bit:
            num1 ^= num
        else:
            num2 ^= num
    return num1, num2

print(find_two_unique([1, 2, 1, 3]))

A No error, outputs (2, 3)

B TypeError

C SyntaxError

D IndexError

💡 Hint

Check the for loop syntax carefully.

The for loop is missing a colon after 'for num in arr', causing a SyntaxError.

❓ Predict Output

advanced

Output of XOR unique elements with negative numbers

What is the output of this code that finds two unique elements in an array containing negative numbers?

DSA Python

def find_two_unique(arr):
    xor_all = 0
    for num in arr:
        xor_all ^= num
    set_bit = xor_all & (-xor_all)
    num1 = 0
    num2 = 0
    for num in arr:
        if num & set_bit:
            num1 ^= num
        else:
            num2 ^= num
    return num1, num2

result = find_two_unique([-1, -2, -1, -3, -2, -4])
print(result)

A (-4, -3)

B (-3, -4)

C (-1, -2)

D (0, 0)

💡 Hint

XOR works with negative numbers as well.

The unique elements are -3 and -4. The function returns them in the order (-3, -4).

🚀 Application

expert

Number of items in dictionary after XOR grouping

After running the following code, how many keys will the dictionary 'groups' contain?

DSA Python

arr = [10, 12, 10, 15, 12, 20]
xor_all = 0
for num in arr:
    xor_all ^= num
set_bit = xor_all & (-xor_all)
groups = {}
for num in arr:
    key = 'group1' if num & set_bit else 'group2'
    groups[key] = groups.get(key, 0) + 1
print(len(groups))

A 2

B 1

C 0

D 3

💡 Hint

There are two groups based on the set bit.

The dictionary 'groups' will have two keys: 'group1' and 'group2', so length is 2.