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)

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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)

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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.