Array Reversal Techniques in DSA Javascript - Time & Space Complexity

Time Complexity: Array Reversal Techniques

O(n)

Understanding Time Complexity

We want to understand how the time to reverse an array changes as the array gets bigger.

How many steps does it take to flip the order of items?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const reverseArray = (arr) => {
  let left = 0;
  let right = arr.length - 1;
  while (left < right) {
    const temp = arr[left];
    arr[left] = arr[right];
    arr[right] = temp;
    left++;
    right--;
  }
  return arr;
};
    

This code reverses an array by swapping elements from the start and end moving towards the center.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Swapping two elements in the array.
• How many times: About half the length of the array (n/2) times.

How Execution Grows With Input

As the array gets bigger, the number of swaps grows roughly in direct proportion to its size.

Input Size (n)	Approx. Operations (Swaps)
10	5
100	50
1000	500

Pattern observation: Doubling the array size roughly doubles the number of swaps needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to reverse grows linearly with the array size; bigger arrays take proportionally more time.

Common Mistake

[X] Wrong: "Reversing an array takes constant time because we just swap elements."

[OK] Correct: Each swap is a step, and the number of swaps grows with the array size, so time grows too.

Interview Connect

Understanding how simple loops affect time helps you explain and optimize code clearly in interviews.

Self-Check

"What if we used recursion to reverse the array? How would the time complexity change?"

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