Demystifying Big O Notation: Understanding Time and Space Complexity

3 min readNov 1, 2023


Photo by Pierre Bamin on Unsplash

Big O notation, time complexity, and space complexity are fundamental concepts in computer science that play a crucial role in analyzing and optimizing algorithms. These concepts provide a structured way to measure and compare the performance of algorithms and data structures. In this post, we will dive into the world of Big O notation, exploring its significance, and gaining a deeper understanding of time and space complexity.

Big O Notation

Big O notation, often referred to as “order of” notation, is a mathematical tool used to describe the upper bound of an algorithm’s growth rate concerning its input size. It is represented as O(f(n)), where f(n) is a function that characterizes the number of operations required by the algorithm relative to the input size (n). Big O notation provides a concise way to assess the efficiency of an algorithm while abstracting away constant factors and lower-order terms.

Common Big O complexities:

  • O(1): Constant time complexity — the algorithm’s runtime is independent of the input size.
  • O(log n): Logarithmic time complexity — the algorithm’s runtime grows slowly as the input size increases.
  • O(n): Linear time complexity — the runtime grows proportionally to the input size.
  • O(n log n): Linearithmic time complexity — often seen in efficient sorting algorithms like merge sort.
  • O(n²), O(n³), …: Polynomial time complexity — common in algorithms with nested loops.
  • O(2^n), O(3^n), …: Exponential time complexity — typically indicates very inefficient algorithms.

Time Complexity

Time complexity is a measure of the amount of time an algorithm takes to complete, expressed in terms of Big O notation. It helps us analyze and compare algorithms to choose the most efficient one for a particular task. Algorithms with lower time complexities are generally preferred, especially for large datasets.

For example, if you have two algorithms for sorting a list, one with O(n²) time complexity and another with O(n log n) time complexity, the latter will be more efficient for larger datasets because it scales better with increasing input size.

Space Complexity

Space complexity, also expressed in Big O notation, quantifies the amount of memory an algorithm uses relative to its input size. It helps in evaluating the memory efficiency of algorithms and is crucial in environments with limited memory resources, such as embedded systems or mobile devices.

Optimizing space complexity often involves trading off memory usage for increased time complexity and vice versa. Balancing these trade-offs is essential when designing algorithms and data structures.

For example, a recursive algorithm may have a lower time complexity but higher space complexity due to the recursion stack, while an iterative solution may use less memory but have a higher time complexity.


Big O notation, time complexity, and space complexity are foundational concepts in computer science that guide algorithm design and analysis. They allow us to evaluate the efficiency and scalability of algorithms, helping us make informed decisions about which algorithms to use in different scenarios.

Understanding these concepts is essential for software developers and computer scientists, as it empowers them to write more efficient code, choose the right data structures, and optimize algorithm performance. Whether you’re a seasoned professional or just starting in the world of computer science, these concepts are invaluable tools in your toolkit for creating efficient and scalable software solutions.