Array Deletion at End in DSA Go - Time & Space Complexity

Time Complexity: Array Deletion at End

O(1)

Understanding Time Complexity

We want to understand how fast it is to remove an item from the end of an array.

How does the time needed change as the array gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

package main

func deleteAtEnd(arr []int) []int {
    if len(arr) == 0 {
        return arr
    }
    return arr[:len(arr)-1]
}

This code removes the last element from the array by slicing it to exclude the last item.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Slicing the array to exclude the last element.
• How many times: This operation happens once per deletion.

How Execution Grows With Input

Removing the last element does not require checking or moving other elements.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The number of operations stays the same no matter how big the array is.

Final Time Complexity

Time Complexity: O(1)

This means removing the last item takes the same small amount of time, no matter how big the array is.

Common Mistake

[X] Wrong: "Removing the last element takes longer if the array is bigger because it has to move all elements."

[OK] Correct: Actually, removing from the end just changes the array length; it does not move or check other elements.

Interview Connect

Knowing that deleting from the end is very fast helps you choose the right data structure and operation in real problems.

Self-Check

"What if we changed to deleting an element from the start of the array? How would the time complexity change?"

Practice

1. What happens when you delete an element from the end of an array in Go?

easy

A. The last element is removed and the array size decreases by one.

B. The first element is removed and the array size decreases by one.

C. The array size stays the same but the last element becomes zero.

D. The array is completely cleared and becomes empty.

Solution

1. Step 1: Understand array deletion at end

   Deleting at the end means removing the last element only, not the first or all.

2. Step 2: Effect on array size

   After removal, the array size decreases by one because one element is gone.

3. Final Answer:

   The last element is removed and the array size decreases by one. -> Option A

4. Quick Check:

   Deleting end element reduces size by one [OK]

Hint: Deleting at end removes last element and shrinks array [OK]

Common Mistakes:

• Thinking first element is removed
• Assuming array size stays same
• Believing array clears completely

2. Which of the following Go code snippets correctly deletes the last element from a slice named arr?

easy

A. arr = arr[len(arr):]

B. arr = arr[:len(arr)-1]

C. arr = arr[1:]

D. arr = arr[:len(arr)+1]

Solution

1. Step 1: Understand slice slicing syntax

   To remove last element, slice up to length minus one: arr[:len(arr)-1].

2. Step 2: Check other options

   arr[len(arr):] results in empty slice, arr[1:] removes first element, arr[:len(arr)+1] causes runtime error.

3. Final Answer:

   arr = arr[:len(arr)-1] -> Option B

4. Quick Check:

   Slice up to len-1 removes last element [OK]

Hint: Use arr[:len(arr)-1] to drop last element [OK]

Common Mistakes:

• Using arr[1:] removes first element
• Using arr[len(arr):] gives empty slice
• Using arr[:len(arr)+1] causes error

3. What is the output of this Go code?

arr := []int{10, 20, 30, 40}
arr = arr[:len(arr)-1]
fmt.Println(arr)

medium

A. [10 20 30 40]

B. []

C. [10 20 30]

D. [20 30 40]

Solution

1. Step 1: Initial array and slicing

   arr starts as [10 20 30 40]. Slicing arr[:len(arr)-1] means arr[:3].

2. Step 2: Result after slicing

   arr[:3] gives first three elements: [10 20 30].

3. Final Answer:

   [10 20 30] -> Option C

4. Quick Check:

   Slice up to len-1 excludes last element [OK]

Hint: Slicing arr[:len(arr)-1] drops last element [OK]

Common Mistakes:

• Expecting last element to remain
• Confusing arr[1:] with arr[:len(arr)-1]
• Thinking output is empty slice

4. Identify the error in this Go code that tries to delete the last element from arr:

arr := []int{1, 2, 3}
arr = arr[:len(arr)]
fmt.Println(arr)

medium

A. It deletes all elements making the array empty.

B. It causes a runtime panic due to invalid slice bounds.

C. It deletes the first element instead of the last.

D. It does not delete any element; array remains unchanged.

Solution

1. Step 1: Analyze slicing bounds

   arr[:len(arr)] means slice from start to full length, so no element is removed.

2. Step 2: Effect on array

   Since slice covers entire array, arr remains [1 2 3] unchanged.

3. Final Answer:

   It does not delete any element; array remains unchanged. -> Option D

4. Quick Check:

   Slicing arr[:len(arr)] keeps full array [OK]

Hint: arr[:len(arr)] keeps full array, no deletion [OK]

Common Mistakes:

• Assuming last element is deleted
• Expecting runtime error
• Thinking first element is removed

5. Given a slice arr := []int{5, 10, 15, 20, 25}, write the Go code to delete the last two elements and print the resulting slice.

hard

A. arr = arr[:len(arr)-2]
fmt.Println(arr)

B. arr = arr[2:]
fmt.Println(arr)

C. arr = arr[:len(arr)-1]
fmt.Println(arr)

D. arr = arr[1:len(arr)-1]
fmt.Println(arr)

Solution

1. Step 1: Understand deletion of last two elements

   To remove last two elements, slice up to length minus two: arr[:len(arr)-2].

2. Step 2: Apply slicing and print

   arr[:3] gives [5 10 15]. Printing arr shows these three elements.

3. Final Answer:

   arr = arr[:len(arr)-2] fmt.Println(arr) -> Option A

4. Quick Check:

   Slice up to len-2 removes last two elements [OK]

Hint: Use arr[:len(arr)-2] to remove last two elements [OK]

Common Mistakes:

• Using arr[2:] removes first two elements
• Removing only last element instead of two
• Incorrect slice bounds causing errors