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

Recall & Review

beginner

What is the main idea behind reversing an array?

Reversing an array means changing the order of elements so the first becomes last, the second becomes second last, and so on, like flipping a row of books from left to right.

beginner

How does the two-pointer technique work for array reversal?

Use two pointers: one at the start and one at the end. Swap the elements they point to, then move the start pointer forward and the end pointer backward until they meet or cross.

intermediate

Show the TypeScript code snippet for reversing an array using a for loop and swapping elements.

const reverseArray = (arr: number[]): number[] => {
  for (let i = 0; i < Math.floor(arr.length / 2); i++) {
    const temp = arr[i];
    arr[i] = arr[arr.length - 1 - i];
    arr[arr.length - 1 - i] = temp;
  }
  return arr;
};

beginner

What is a simple built-in method in TypeScript/JavaScript to reverse an array?

Use the built-in method arr.reverse() which reverses the array in place, changing the order of elements from last to first.

intermediate

Why is it important to only swap elements up to the middle of the array when reversing?

Swapping beyond the middle would undo previous swaps and return the array to original order. Stopping at the middle ensures each pair is swapped only once.

Which technique uses two pointers to reverse an array?

A Sorting the array in descending order

B Using recursion to reverse

C Start and end pointers swapping elements

D Using a stack to push and pop elements

The two-pointer technique places one pointer at the start and one at the end, swapping elements until they meet.

What does the built-in method arr.reverse() do?

A Reverses the array in place

B Sorts the array in ascending order

C Creates a new reversed array without changing original

D Removes duplicates from the array

arr.reverse() reverses the array in place, changing the order of elements.

When swapping elements to reverse an array, how many swaps are needed?

A Equal to the array length

B Double the array length

C One less than the array length

D Half the array length

Only half the array length swaps are needed because each swap fixes two elements.

What happens if you swap elements beyond the middle of the array during reversal?

A Array becomes empty

B Reversal is undone, original order restored

C Array gets sorted

D No change to array

Swapping beyond the middle reverses previous swaps, undoing the reversal.

Which of these is NOT a way to reverse an array?

A Using array.sort() method

B Using a for loop with swapping

C Using two pointers swapping elements

D Using arr.reverse() method

array.sort() sorts elements but does not reverse the array.

Explain how to reverse an array using the two-pointer technique.

Think about swapping pairs from outside moving inward.

Describe the difference between reversing an array using a built-in method and manually swapping elements.

Compare ease of use and control over process.

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