How Algorithms Work
Chapter 2 Algorithms Fundamentals

Chapter 2: Fundamentals of Algorithms

In this chapter, we cover the fundamental concepts and data structures that form the foundation for studying algorithms and developing efficient programs. We start by discussing data types and data structures, which provide ways to organize and manipulate data. Then we introduce three essential abstract data types: bags, queues, and stacks. These data types serve as building blocks for more complex algorithms and data structures. We also explore the analysis of algorithms, a crucial aspect of understanding the efficiency and scalability of programs. Finally, we present a case study on the union-find problem, which demonstrates how to apply the concepts learned in this chapter to solve a practical problem.

Data Types and Data Structures

Data types define a set of values and a set of operations that can be performed on those values. Primitive data types, such as integers, floating-point numbers, and booleans, are built into programming languages. However, to solve more complex problems, we often need to create our own data types, known as abstract data types (ADTs).

Data structures, on the other hand, provide ways to organize and store data in a computer's memory. They define how data is arranged and accessed, which can greatly impact the efficiency of algorithms that operate on the data. Some common data structures include arrays, linked lists, trees, and hash tables.

When designing algorithms, it is essential to choose appropriate data types and data structures that support the required operations efficiently. The choice of data structure can make a significant difference in the performance of an algorithm.

Bags, Queues, and Stacks

Bags, queues, and stacks are three fundamental abstract data types that are widely used in algorithm design and programming.


A bag is an unordered collection of elements that allows for duplicates. The main operations supported by a bag are:

  • add(item): Add an item to the bag.
  • isEmpty(): Check if the bag is empty.
  • size(): Return the number of items in the bag.

Bags are useful when we need to keep track of a collection of items without caring about their order or uniqueness.


A queue is a collection of elements that follows the first-in, first-out (FIFO) principle. The main operations supported by a queue are:

  • enqueue(item): Add an item to the end of the queue.
  • dequeue(): Remove and return the item from the front of the queue.
  • isEmpty(): Check if the queue is empty.
  • size(): Return the number of items in the queue.

Queues are often used in scenarios where items need to be processed in the order they arrive, such as task scheduling or breadth-first search.


A stack is a collection of elements that follows the last-in, first-out (LIFO) principle. The main operations supported by a stack are:

  • push(item): Add an item to the top of the stack.
  • pop(): Remove and return the item from the top of the stack.
  • isEmpty(): Check if the stack is empty.
  • size(): Return the number of items in the stack.

Stacks are commonly used in algorithms that require backtracking or reversing the order of elements, such as depth-first search or expression evaluation.

Bags, queues, and stacks can be implemented using various data structures, such as arrays or linked lists, depending on the specific requirements of the application.

Analysis of Algorithms

Analyzing the efficiency of algorithms is crucial for understanding their performance characteristics and making informed decisions when choosing algorithms for specific problems. The analysis of algorithms involves studying the time complexity and space complexity of an algorithm.

Time complexity refers to the amount of time an algorithm takes to solve a problem as a function of the input size. It is usually expressed using the big-O notation, which provides an upper bound on the growth rate of the algorithm's running time. For example, an algorithm with a time complexity of O(n) means that its running time grows linearly with the input size.

Space complexity, on the other hand, refers to the amount of memory an algorithm requires to solve a problem as a function of the input size. It is also expressed using the big-O notation and indicates how much additional memory an algorithm needs as the input size grows.

When analyzing algorithms, we often consider the worst-case, average-case, and best-case scenarios. The worst-case scenario represents the maximum time or space required by an algorithm for any input of a given size, while the best-case scenario represents the minimum time or space required. The average-case scenario represents the expected time or space required for a typical input.

It is important to note that the actual running time of an algorithm depends on various factors, such as the programming language, hardware, and compiler optimizations. However, the big-O notation provides a standardized way to compare the efficiency of different algorithms independent of these factors.

Case Study: Union-Find

The union-find problem, also known as the disjoint-set data structure, is a classic example that demonstrates the application of the concepts discussed in this chapter. The problem involves maintaining a collection of disjoint sets and supporting two main operations:

  • union(p, q): Merge the sets containing elements p and q.
  • find(p): Find the set containing element p.

The union-find data structure has numerous applications, such as detecting cycles in graphs, finding connected components, and solving problems related to percolation and dynamic connectivity.

There are several algorithms for implementing the union-find data structure, each with different trade-offs between the time complexity of the union and find operations. Some common implementations include:

  • Quick-find: The find operation is constant time, but the union operation takes linear time.
  • Quick-union: The union operation is faster than quick-find, but the find operation can take linear time in the worst case.
  • Weighted quick-union: An improvement over quick-union that balances the trees by size, making both union and find operations logarithmic in the worst case.
  • Weighted quick-union with path compression: A further optimization that flattens the tree structure during find operations, resulting in nearly constant time complexity for both union and find.

The union-find case study highlights the importance of choosing appropriate data structures and algorithms based on the problem requirements and performance considerations.


In this chapter, we explored the fundamental concepts of data types, data structures, and abstract data types, focusing on bags, queues, and stacks. We also discussed the analysis of algorithms and the importance of considering time and space complexity when designing and selecting algorithms. The union-find case study demonstrated how these concepts can be applied to solve real-world problems efficiently.

As we progress through the book, we will build upon these foundational concepts and explore more advanced algorithms and data structures. Understanding the principles covered in this chapter will provide a solid basis for tackling more complex problems and designing efficient solutions.