An array is a collection of items stored at contiguous memory locations. It's the most widely used data structure because of its simplicity and constant-time access.

Think of an array as a row of lockers in a school hallway:

1. Every locker has a number (index), starting from 0.
2. If you know the locker number, you can walk straight to it and open it.
3. If you want to put a new locker in the middle, you have to move all the other lockers to the right to make space.

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Core Properties

• Random Access: You can access any element in $O(1)$ time if you know the index.
• Fixed Size (Static): In some languages (C++, Java), arrays have a fixed size. In JavaScript, arrays are dynamic and can grow.
• Contiguous Memory: Elements are placed right next to each other, which makes them very fast for the CPU to read.

Basic Operations (Complexity)

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javascript Standard
// Initialization
const arr = [10, 20, 30, 40, 50];

// 1. Accessing
console.log(arr[2]); // 30 (O(1))

// 2. Iterating
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

// 3. Common Interview Task: Reverse an Array
function reverseArray(arr) {
  let start = 0;
  let end = arr.length - 1;
  
  while (start < end) {
    // Swap elements using destructuring
    [arr[start], arr[end]] = [arr[end], arr[start]];
    start++;
    end--;
  }
  return arr;
}

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Top "In-Place" Tricks

Interviews often ask you to modify the array without using extra space ($O(1)$ space).

• Two Pointers: Use two pointers (start/end or slow/fast) to swap or compare elements.
• Index-based Hashing: Use the array indexes as keys (if the values are within the range of array size).

Questions to expect

• Find the maximum/minimum element.
• Check if an array is sorted.
• Find the second largest element.
• Move all zeros to the end of the array.