Data Structure

A data structure is a collection of data values, the relationships among them, and the operations that can be performed on those values. It is the organizational layer that algorithms operate on.

Formal View

A data structure is an algebraic structure about data, defined by:

A set of values — the data being stored
Relations — how values are connected (e.g. ordering, parent-child, key-value)
Operations — what you can do (insert, delete, search, traverse)

The choice of data structure determines the time and space complexity of the operations an algorithm needs.

Fundamental Structures

Arrays

Fixed-size, contiguous memory
$O(1)$ random access by index
$O(n)$ insertion/deletion (requires shifting)
Cache-friendly — spatial locality

Linked Lists

Dynamic size, nodes connected by pointers
$O(1)$ insertion/deletion at known node
$O(n)$ access by position
Poor cache locality

Hash Tables

Key-value store via hash function
$O(1)$ average for insert, delete, lookup
$O(n)$ worst-case (hash collisions)
No inherent ordering

Trees

Variant	Search	Insert	Delete	Notes
BST (unbalanced)	$O(n)$ worst	$O(n)$ worst	$O(n)$ worst	Degenerates on sorted input
AVL Tree	$O(\log n)$	$O(\log n)$	$O(\log n)$	Height-balanced
Red-Black Tree	$O(\log n)$	$O(\log n)$	$O(\log n)$	Used in Linux scheduler
B-Tree	$O(\log n)$	$O(\log n)$	$O(\log n)$	Optimized for disk I/O
Heap	$O(n)$	$O(\log n)$	$O(\log n)$	$O(1)$ min/max access

Graphs

General structure: vertices $V$ and edges $E$ .

Adjacency matrix — $O(1)$ edge lookup, $O(V^2)$ space
Adjacency list — $O(V + E)$ space, better for sparse graphs

Choosing a Data Structure

The right structure depends on the dominant operations:

Mostly lookups by key → hash table
Ordered traversal + balanced ops → balanced BST
Priority queue / scheduling → heap
Graph traversal (BFS, DFS) → adjacency list
Time-series / sliding window → deque or circular buffer

Algorithm — uses data structures to process inputs
RAM Model of Computation — the machine model that determines operation costs
Computational Problem — the problem a data structure helps solve