Array Reversal Techniques in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does reversing an array mean?

Reversing an array means changing the order of elements so that the first element becomes the last, the second becomes the second last, and so on.

beginner

How can you reverse an array using a built-in JavaScript method?

You can use the array.reverse() method, which reverses the elements of the array in place.

intermediate

Explain the two-pointer technique for reversing an array.

The two-pointer technique uses two indexes: one starting at the beginning and one at the end of the array. Swap the elements at these pointers and move them towards the center until they meet or cross.

intermediate

What is the time complexity of reversing an array using the two-pointer method?

The time complexity is O(n), where n is the number of elements in the array, because each element is visited once during swapping.

intermediate

Why might you choose to reverse an array manually instead of using array.reverse()?

Manual reversal allows more control, such as reversing only part of the array or avoiding modifying the original array by creating a new reversed copy.

Which JavaScript method reverses an array in place?

A array.reverse()

B array.sort()

C array.push()

D array.slice()

The array.reverse() method reverses the elements of the array in place.

In the two-pointer technique, where do the pointers start?

A One at the start, one at the end

B Both at the end

C Both at the start

D One in the middle, one at the end

One pointer starts at the beginning and the other at the end of the array.

What is the time complexity of reversing an array using the two-pointer method?

A O(n^2)

B O(1)

C O(n)

D O(log n)

Reversing an array with two pointers takes O(n) time because each element is swapped once.

Which of these is NOT a reason to reverse an array manually?

A To reverse only part of the array

B To avoid modifying the original array

C To have more control over the process

D To use less memory than array.reverse()

Manual reversal usually uses similar or more memory; it is not chosen to save memory compared to array.reverse().

What will be the output after reversing [1, 2, 3, 4] using array.reverse()?

A [1, 2, 3, 4]

B [4, 3, 2, 1]

C [2, 1, 4, 3]

D [3, 4, 1, 2]

The array elements are reversed in place, so the output is [4, 3, 2, 1].

Describe how to reverse an array using the two-pointer technique step-by-step.

Think about swapping pairs from outside moving inward.

Explain the difference between using array.reverse() and manually reversing an array.

Consider control and side effects on the original array.

Practice

1. What does the reverse() method do to an array in JavaScript?

easy

A. It removes duplicate elements from the array.

B. It sorts the array in ascending order.

C. It reverses the order of elements in the original array.

D. It creates a new array with elements doubled.

Solution

1. Step 1: Understand the reverse() method

   The reverse() method changes the original array by reversing the order of its elements.

2. Step 2: Confirm the effect on the original array

   It does not create a new array or sort; it simply reverses the existing array in place.

3. Final Answer:

   It reverses the order of elements in the original array. -> Option C

4. Quick Check:

   reverse() = reverses array order [OK]

Hint: reverse() flips array order in place [OK]

Common Mistakes:

• Thinking reverse() sorts the array
• Assuming reverse() creates a new array
• Confusing reverse() with removing duplicates

2. Which of the following is the correct syntax to reverse an array named arr in JavaScript?

easy

A. arr.reverse();

B. reverse(arr);

C. arr = reverse();

D. arr.reverse(arr);

Solution

1. Step 1: Identify the method call syntax

   The reverse() method is called on the array object itself, so the syntax is arr.reverse();.

2. Step 2: Check other options for errors

   Options A, B, and C are incorrect because reverse is not a standalone function and does not take arguments.

3. Final Answer:

   arr.reverse(); -> Option A

4. Quick Check:

   Method called on array object = arr.reverse() [OK]

Hint: Use arrayName.reverse() to reverse in place [OK]

Common Mistakes:

• Calling reverse as a standalone function
• Passing the array as an argument to reverse
• Assigning reverse() without calling on array

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

const arr = [1, 2, 3, 4];
const reversed = arr.reverse();
console.log(reversed);

medium

A. undefined

B. [1, 2, 3, 4]

C. [1, 4, 3, 2]

D. [4, 3, 2, 1]

Solution

1. Step 1: Understand reverse() return value

   The reverse() method reverses the original array and returns the same reversed array.

2. Step 2: Trace the code output

   After calling arr.reverse(), arr becomes [4, 3, 2, 1], and reversed holds this reversed array. Logging reversed prints [4, 3, 2, 1].

3. Final Answer:

   [4, 3, 2, 1] -> Option D

4. Quick Check:

   reverse() returns reversed array [OK]

Hint: reverse() returns the reversed array itself [OK]

Common Mistakes:

• Expecting original array unchanged
• Thinking reverse() returns undefined
• Confusing reverse() with sorting

4. Find the error in the following code that tries to reverse an array nums:

function reverseArray(nums) {
  for(let i = 0; i < nums.length / 2; i++) {
    let temp = nums[i];
    nums[i] = nums[nums.length - i];
    nums[nums.length - i] = temp;
  }
  return nums;
}

medium

A. The index nums.length - i is out of bounds; should be nums.length - i - 1.

B. The temp variable is not needed for swapping.

C. The loop should run while i < nums.length / 2, not <=.

D. The function should return a new array, not modify nums.

Solution

1. Step 1: Check index calculations for swapping

   The code uses nums.length - i as the index for the element to swap, but array indices go from 0 to nums.length - 1, so it should be nums.length - i - 1.

2. Step 2: Confirm loop condition and swapping logic

   The loop condition i < nums.length / 2 is acceptable, and temp is needed for swapping. Modifying the original array is allowed here.

3. Final Answer:

   The index nums.length - i is out of bounds; should be nums.length - i - 1. -> Option A

4. Quick Check:

   Array indices start at 0, so subtract 1 [OK]

Hint: Array last index = length - 1, not length [OK]

Common Mistakes:

• Using nums.length - i instead of nums.length - i - 1
• Removing temp variable causing swap errors
• Incorrect loop boundary causing extra swaps

5. Given an array arr, which code snippet correctly reverses the array without modifying the original array?

hard

A. const reversed = arr.reverse();

B. const reversed = arr.slice().reverse();

C. const reversed = [...arr].sort().reverse();

D. const reversed = arr.reverse().map(x => x);

Solution

1. Step 1: Understand how to avoid modifying original array

   Using arr.slice() creates a shallow copy of the array. Calling reverse() on this copy reverses it without changing the original.

2. Step 2: Analyze other options

   const reversed = arr.reverse(); reverses the original array. const reversed = [...arr].sort().reverse(); sorts before reversing, changing order. const reversed = arr.reverse().map(x => x); reverses the original array because reverse() is called on arr before mapping.

3. Final Answer:

   const reversed = arr.slice().reverse(); -> Option B

4. Quick Check:

   Copy then reverse to keep original safe [OK]

Hint: Use slice() to copy before reverse() [OK]

Common Mistakes:

• Using reverse() directly modifies original array
• Sorting before reversing changes order
• Thinking reverse().map(x => x) preserves original array