Remove Duplicates from Sorted Array Two Pointer in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the two-pointer approach to remove duplicates from a sorted array?

Use one pointer to track the position of unique elements and another to scan through the array. When a new unique element is found, place it at the unique pointer's position and move the pointer forward.

beginner

Why does the two-pointer method work efficiently on a sorted array?

Because duplicates are adjacent in a sorted array, the method can easily detect duplicates by comparing current and previous elements, avoiding extra space and multiple passes.

beginner

In the two-pointer approach, what does the 'slow' pointer represent?

The 'slow' pointer marks the position where the next unique element should be placed, effectively tracking the length of the array without duplicates.

beginner

What is the time complexity of removing duplicates from a sorted array using the two-pointer technique?

O(n), where n is the number of elements in the array, because each element is visited at most once.

intermediate

What happens to the elements after the 'slow' pointer in the array after duplicates are removed?

They remain in the array but are not considered part of the unique elements. The array length is effectively the position of the 'slow' pointer.

What does the 'fast' pointer do in the two-pointer approach to remove duplicates?

A Sorts the array

B Marks the position of unique elements

C Scans through the array to find unique elements

D Deletes duplicates directly

The 'fast' pointer moves through the array to find unique elements by comparing with the previous element.

Why is the array required to be sorted for this two-pointer method to work correctly?

A Because duplicates are grouped together

B Because sorting removes duplicates automatically

C Because the method uses hashing

D Because the array size must be fixed

Duplicates appear next to each other in a sorted array, making it easy to detect and skip them.

What is the final output of the two-pointer duplicate removal method?

A Length of unique elements and modified array with unique elements at the front

B Sorted array with duplicates

C Array with duplicates removed and empty spaces removed

D Array reversed

The method returns the count of unique elements and the array modified so that unique elements are at the start.

If the input array is [1,1,2], what will be the array after removing duplicates using two pointers?

A [1,2,2]

B [1,1,2]

C [2,1,_]

D [1,2,_]

The unique elements 1 and 2 are placed at the start; the third position is ignored.

What is the space complexity of the two-pointer approach for removing duplicates from a sorted array?

A O(n)

B O(1)

C O(log n)

D O(n^2)

The method uses constant extra space by modifying the array in place.

Explain step-by-step how the two-pointer technique removes duplicates from a sorted array.

Think about how you move through the array and keep track of unique values.

Describe the state of the array and pointers after processing the input [0,0,1,1,1,2,2,3,3,4] with the two-pointer method.

Visualize the array as you move pointers and place unique elements.