Array Reversal Techniques in DSA Typescript - Time & Space Complexity

Time Complexity: Array Reversal Techniques

O(n)

Understanding Time Complexity

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

How does the number of steps grow when we reverse more items?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function reverseArray(arr: number[]): number[] {
  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 the elements of an array by swapping pairs from the ends moving toward the center.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Swapping two elements inside a while loop.
• 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 the size.

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

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 in a straight line with the number of items in the array.

Common Mistake

[X] Wrong: "Reversing an array takes the same time no matter how big it is."

[OK] Correct: The code must swap pairs for each element, so bigger arrays need more swaps and more time.

Interview Connect

Knowing how array reversal scales helps you explain your code clearly and choose the best approach in real tasks.

Self-Check

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

Practice

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

easy

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

B. It sorts the array in ascending order.

C. It removes duplicate elements from the array.

D. It creates a new array with elements doubled.

Solution

1. Step 1: Understand the reverse() method

   The reverse() method changes the order of elements in the array so the last element becomes first and the first becomes last.

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

   This method modifies the original array itself, not creating a new one.

3. Final Answer:

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

4. Quick Check:

   reverse() reverses array order [OK]

Hint: Remember: reverse() flips the array 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 TypeScript?

easy

A. arr->reverse();

B. reverse(arr);

C. arr = reverse();

D. arr.reverse();

Solution

1. Step 1: Identify the method call on the array

   In TypeScript, reverse() is a method called directly on the array object using dot notation.

2. Step 2: Verify syntax

   arr.reverse(); is correct. reverse(arr); incorrectly treats reverse as a standalone function. arr = reverse(); misuses the method. arr->reverse(); uses invalid syntax.

3. Final Answer:

   arr.reverse(); -> Option D

4. Quick Check:

   Use dot notation: arr.reverse() [OK]

Hint: Use dot notation: array.reverse() [OK]

Common Mistakes:

• Calling reverse() as a standalone function
• Assigning reverse() without calling on array
• Using incorrect symbols like '->'

3. What is the output of the following TypeScript code?

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

medium

A. [4, 3, 2, 1]

B. [1, 2, 3, 4]

C. [1, 4, 3, 2]

D. Error: reverse is not a function

Solution

1. Step 1: Understand the initial array

   The array starts as [1, 2, 3, 4].

2. Step 2: Apply the reverse() method

   Calling arr.reverse() reverses the array in place, changing it to [4, 3, 2, 1].

3. Final Answer:

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

4. Quick Check:

   reverse() flips array order [OK]

Hint: reverse() changes array order from end to start [OK]

Common Mistakes:

• Expecting original order after reverse()
• Confusing reverse() with sorting
• Thinking reverse() returns a new array

4. Identify the error in this TypeScript code that tries to reverse an array:

const numbers = [5, 6, 7];
const reversed = numbers.reverse;
console.log(reversed);

medium

A. reverse is not a valid method on arrays.

B. Missing parentheses to call reverse method.

C. reverse method should be called with an argument.

D. Array cannot be reversed using reverse method.

Solution

1. Step 1: Check how reverse is used

   The code assigns numbers.reverse without parentheses, so it assigns the function itself, not the reversed array.

2. Step 2: Correct usage of reverse()

   To reverse the array, the method must be called with parentheses: numbers.reverse().

3. Final Answer:

   Missing parentheses to call reverse method. -> Option B

4. Quick Check:

   Call reverse() with () to execute [OK]

Hint: Always use () to call methods like reverse() [OK]

Common Mistakes:

• Forgetting parentheses when calling methods
• Thinking reverse needs arguments
• Believing reverse is not a valid method

5. Given the array const data = [10, 20, 30, 40, 50];, which code snippet correctly reverses the array without modifying the original array?

hard

A. const reversed = data.reverse();

B. const reversed = data.sort().reverse();

C. const reversed = [...data].reverse();

D. const reversed = data.map().reverse();

Solution

1. Step 1: Understand the problem requirement

   We want to reverse the array but keep the original array unchanged.

2. Step 2: Analyze each option

   const reversed = data.reverse(); reverses the original array directly, modifying it.
   const reversed = [...data].reverse(); creates a shallow copy using spread [...data] and then reverses the copy, preserving the original.
   const reversed = data.sort().reverse(); sorts the original array first, changing order before reversing, and modifies original.
   const reversed = data.map().reverse(); uses map() incorrectly without a callback, causing error.

3. Final Answer:

   const reversed = [...data].reverse(); -> Option C

4. Quick Check:

   Copy then reverse to keep original safe [OK]

Hint: Copy array first with [...array] before reverse() [OK]

Common Mistakes:

• Using reverse() directly modifies original array
• Sorting before reversing changes order incorrectly
• Using map() without a callback causes error