🧠 Solve LeetCode: Reverse an Array

✅ Problem Statement

Given an array arr, reverse its elements in-place or return a new reversed array.

This is a fundamental problem to understand array manipulation, two-pointer techniques, and space optimization.

🧩 Example

Input: arr = [1, 2, 3, 4, 5]
Output: [5, 4, 3, 2, 1]

🔍 Approach 1: Brute Force (O(n) time, O(n) space)

What: Create a new array and copy elements in reverse order. Why: Simple, intuitive, but uses extra space. How: Start from the last element of the original array and append it to a new array.

🐍 Python Code

def reverse_array_brute(arr):
    reversed_arr = []
    for i in range(len(arr)-1, -1, -1):
        reversed_arr.append(arr[i])
    return reversed_arr
 
# Example
arr = [1, 2, 3, 4, 5]
print(reverse_array_brute(arr))  # Output: [5, 4, 3, 2, 1]

🧪 JavaScript Code

function reverseArrayBrute(arr) {
  const reversed = [];
  for (let i = arr.length - 1; i >= 0; i--) {
    reversed.push(arr[i]);
  }
  return reversed;
}
 
// Example
const arr = [1, 2, 3, 4, 5];
console.log(reverseArrayBrute(arr)); // Output: [5, 4, 3, 2, 1]

Drawback: Uses extra O(n) space, not ideal for large arrays.

⚡ Approach 2: Two-Pointer In-Place (O(n) time, O(1) space)

What: Swap elements from start and end moving towards the middle. Why: Reverses the array in-place, no extra space needed. How: Use two pointers: left at start, right at end. Swap until left >= right.

🐍 Python Code

def reverse_array_inplace(arr):
    left, right = 0, len(arr) - 1
    while left < right:
        arr[left], arr[right] = arr[right], arr[left]
        left += 1
        right -= 1
    return arr
 
# Example
arr = [1, 2, 3, 4, 5]
print(reverse_array_inplace(arr))  # Output: [5, 4, 3, 2, 1]

🧪 JavaScript Code

function reverseArrayInPlace(arr) {
  let left = 0,
    right = arr.length - 1;
  while (left < right) {
    [arr[left], arr[right]] = [arr[right], arr[left]];
    left++;
    right--;
  }
  return arr;
}
 
// Example
const arr = [1, 2, 3, 4, 5];
console.log(reverseArrayInPlace(arr)); // Output: [5, 4, 3, 2, 1]

Execution Trace / Step-by-Step

arr = [1, 2, 3, 4, 5]
left=0, right=4 → swap arr[0] & arr[4] → [5,2,3,4,1]
left=1, right=3 → swap arr[1] & arr[3] → [5,4,3,2,1]
left=2, right=2 → stop, middle reached

🔍 Breaking It Down for Understanding

1. Two-Pointer Concept: Maintain left and right to perform swaps efficiently.
2. In-Place Advantage: Saves space, important for memory-sensitive problems.
3. Mental Model: Imagine folding the array from both ends until you meet in the middle.
4. Pattern Recognition: Many array problems use two-pointer or in-place swapping techniques.

📝 Summary

Tip: Whenever you need to reverse or rearrange arrays, think about two-pointer swaps before creating new arrays. It’s usually more optimal.