Two Pointer Technique on Arrays in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the left pointer at the start of the array.

DSA Python

arr = [1, 3, 5, 7, 9]
left = [1]
right = len(arr) - 1
print(left, right)

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

A1

Blen(arr)

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 or -1 instead of 0.

The left pointer should start at index 0, which is the first element of the array.

2fill in blank

medium

Complete the code to move the right pointer one step to the left.

DSA Python

right = 5
right = right [1] 1
print(right)

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of - to move left.

To move the right pointer one step left, subtract 1 from it.

3fill in blank

hard

Fix the error in the loop condition to stop when left pointer crosses right pointer.

DSA Python

left = 0
right = 4
while left [1] right:
    print(left, right)
    left += 1
    right -= 1

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

A<

B>

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= or >= causing extra iterations.

The loop should run while left is less than right to avoid crossing pointers.

4fill in blank

hard

Fill both blanks to create a two-pointer loop that swaps elements until pointers meet.

DSA Python

arr = [2, 4, 6, 8, 10]
left = 0
right = len(arr) - 1
while left [1] right:
    arr[left], arr[right] = arr[right], arr[left]
    left = left [2] 1
    right -= 1
print(arr)

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

A<

B+

C-

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes extra swaps.

Decrementing left pointer instead of incrementing.

The loop runs while left is less than right. Left pointer moves right by adding 1.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each element to its index if the element is even.

DSA Python

arr = [1, 2, 3, 4, 5]
even_index_map = [1]: [2] for [3] in range(len(arr)) if arr[i] % 2 == 0
print(even_index_map)

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

Aarr[i]

Bi

Darr

Attempts:

3 left

💡 Hint

Common Mistakes

Using arr instead of arr[i] as key.

Using wrong loop variable.

The comprehension maps each even element (arr[i]) to its index (i). The loop variable is i.

Practice

1. What is the main idea behind the Two Pointer Technique when used on arrays?

easy

A. Use two arrays to store temporary results

B. Sort the array twice before processing

C. Use two indices to scan the array from different ends or positions simultaneously

D. Use recursion to divide the array into two parts

Solution

1. Step 1: Understand the two pointer concept

   The technique uses two pointers (indices) to traverse the array, often starting from opposite ends or different positions.

2. Step 2: Recognize the purpose

   This helps solve problems efficiently by reducing the number of iterations compared to nested loops.

3. Final Answer:

   Use two indices to scan the array from different ends or positions simultaneously -> Option C

4. Quick Check:

   Two pointers scan array ends or positions [OK]

Hint: Think of two fingers moving inward or outward on the array [OK]

Common Mistakes:

• Confusing two pointers with using two separate arrays
• Assuming sorting is always required
• Thinking recursion is part of two pointer technique

2. Which of the following is the correct syntax to initialize two pointers left and right for an array arr in Python?

easy

A. left = 0; right = len(arr) - 1

B. left = 1; right = len(arr)

C. left = -1; right = len(arr)

D. left = 0; right = len(arr)

Solution

1. Step 1: Identify valid pointer positions

   In Python, array indices start at 0, so the left pointer should start at 0.

2. Step 2: Set right pointer correctly

   The right pointer should be at the last valid index, which is length of array minus 1.

3. Final Answer:

   left = 0; right = len(arr) - 1 -> Option A

4. Quick Check:

   Start left at 0 and right at last index [OK]

Hint: Remember Python arrays start at 0, last index is length-1 [OK]

Common Mistakes:

• Setting right pointer to len(arr) causes index error
• Starting left pointer at 1 misses first element
• Using negative indices incorrectly

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

arr = [1, 3, 5, 7, 9]
left, right = 0, 4
result = []
while left < right:
    result.append(arr[left] + arr[right])
    left += 1
    right -= 1
print(result)

medium

A. [10, 14]

B. [10, 12]

C. [10, 12, 14]

D. [10, 10]

Solution

1. Step 1: Trace the loop iterations

   Initial pointers: left=0, right=4 - Iteration 1: arr[0]+arr[4] = 1+9=10, result=[10] - left=1, right=3 - Iteration 2: arr[1]+arr[3] = 3+7=10, result=[10,10] - left=2, right=2 (loop ends because left is not less than right)

2. Step 2: Check loop condition and final result

   Loop stops when left == right (2), so only two sums added: [10, 10]

3. Final Answer:

   [10, 10] -> Option D

4. Quick Check:

   Sum pairs from ends until pointers meet [OK]

Hint: Add pairs from ends until pointers cross [OK]

Common Mistakes:

• Including middle element sum when pointers meet
• Not updating pointers correctly
• Confusing indices causing wrong sums

4. Find the error in the following code that uses two pointers to check if an array is a palindrome:

def is_palindrome(arr):
    left, right = 0, len(arr)
    while left < right:
        if arr[left] != arr[right]:
            return False
        left += 1
        right -= 1
    return True

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

medium

A. left should start at 1

B. right should be initialized as len(arr) - 1

C. Use 'while left <= right' instead of 'while left < right'

D. Return True should be inside the loop

Solution

1. Step 1: Check pointer initialization

   In Python, last index is len(arr) - 1, but code sets right = len(arr), which is out of range.

2. Step 2: Correct pointer initialization

   Change right to len(arr) - 1 to avoid index error and correctly compare elements.

3. Final Answer:

   right should be initialized as len(arr) - 1 -> Option B

4. Quick Check:

   Last index = length - 1 [OK]

Hint: Last index is length minus one, not length [OK]

Common Mistakes:

• Setting right pointer to length causes index error
• Changing loop condition incorrectly
• Returning True too early inside loop

5. Given a sorted array arr and a target sum target, how can you use the two pointer technique to find if there exists two numbers that add up to target? Choose the correct approach.

hard

A. Initialize left at 0 and right at len(arr)-1; move pointers inward based on sum comparison

B. Initialize both pointers at 0 and move right pointer only

C. Sort the array again and use binary search for each element

D. Use two nested loops to check all pairs

Solution

1. Step 1: Set pointers at array ends

   Start left pointer at 0 (start) and right pointer at last index (len(arr)-1).

2. Step 2: Move pointers based on sum

   Calculate sum of arr[left] + arr[right]. If sum equals target, return True. If sum is less, move left pointer right to increase sum. If sum is more, move right pointer left to decrease sum.

3. Final Answer:

   Initialize left at 0 and right at len(arr)-1; move pointers inward based on sum comparison -> Option A

4. Quick Check:

   Two pointers from ends adjust to find target sum [OK]

Hint: Move left/right pointers based on sum compared to target [OK]

Common Mistakes:

• Moving only one pointer without condition
• Using nested loops instead of two pointers
• Not starting pointers at array ends