An Executable Data Structures Cheat Sheet for Interviews

Array

• Quick summary: a collection that stores elements in order and looks them up by index.
• Also known as: fixed array, static array.
• Important facts:
  • Stores elements sequentially, one after another.
  • Each array element has an index. Zero-based indexing is used most often: the first index is 0, the second is 1, and so on.
  • Is created with a fixed size. Increasing or decreasing the size of an array is impossible.
  • Can be one-dimensional (linear) or multi-dimensional.
  • Allocates contiguous memory space for all its elements.
• Pros:
  • Ensures constant time access by index.
  • Constant time append (insertion at the end of an array).
• Cons:
  • Fixed size that can't be changed.
  • Search, insertion and deletion are O(n). After insertion or deletion, all subsequent elements are moved one index further.
  • Can be memory intensive when capacity is underused.
• Notable uses:
  • The String data type that represents text is implemented in programming languages as an array that consists of a sequence of characters plus a terminating character.
• Time complexity (worst case):
  • Access: O(1)
  • Search: O(n)
  • Insertion: O(n) (append: O(1))
  • Deletion: O(n)
• See also:
  • A Gentle Refresher Into Arrays and Strings
  • Interview Problems: Easy Strings
  • Interview Problems: Basic Arrays
  • Interview Problems: Medium Arrays

Arrays are a primitive building block in most languages, here's how to initialize them:

1int[] arr = {1, 2, 3};