# How to calculate algorithm complexity

## What is the complexity of an algorithm?

The complexity of an algorithm refers to the efficiency or performance of the algorithm, usually measured in terms of its time complexity and space complexity.

1. Time Complexity: Time complexity measures how the running time of an algorithm grows as the input size increases. It provides an estimation of the number of operations or steps the algorithm requires to solve a problem. Common notations used to express time complexity include Big O notation (e.g., O(1), O(log n), O(n), O(n^2)).

2. Space Complexity: Space complexity measures the amount of memory or storage space required by an algorithm to solve a problem. It includes both the auxiliary space (extra space for variables, data structures, etc.) and the input space (memory required to store the input data). Similar to time complexity, space complexity is also expressed using Big O notation.

The complexity of an algorithm depends on various factors, such as the algorithm's design, the problem it solves, the input size, and the computational resources available. Analyzing the complexity of an algorithm helps in understanding its efficiency, predicting its performance for different input sizes, and comparing it with other algorithms.

When designing or analyzing algorithms, it is desirable to have algorithms with lower time complexity and space complexity, as they tend to be more efficient and scalable. However, it's important to note that complexity analysis provides a theoretical understanding of an algorithm's performance and may not always reflect the real-world execution time, which can be influenced by hardware, software optimizations, and other practical considerations.

## The asymptotic complexity of algorithms

The asymptotic complexity of algorithms refers to how the performance of an algorithm scales with the input size in the worst-case scenario. It provides an understanding of how the algorithm's running time or space requirements grow as the input size becomes larger.

The most common notation used to express asymptotic complexity is Big O notation. Here are some common asymptotic complexity classes:

• O(1) - Constant time complexity: The algorithm's running time or space requirements remain constant regardless of the input size. Examples include accessing an element in an array or performing a simple arithmetic operation.

• O(log n) - Logarithmic time complexity: The algorithm's running time or space requirements increase logarithmically with the input size. Examples include binary search or certain divide and conquer algorithms that partition the input.

• O(n) - Linear time complexity: The algorithm's running time or space requirements grow linearly with the input size. Examples include traversing an array or performing a simple linear search.

• O(n log n) - Linearithmic time complexity: The algorithm's running time or space requirements grow in a rate that is slightly larger than linear. Examples include efficient sorting algorithms like merge sort or quicksort.

• O(n^2), O(n^3), etc. - Polynomial time complexity: The algorithm's running time or space requirements grow with a polynomial function of the input size. Examples include nested loops or certain brute-force algorithms.

• O(2^n), O(n!), etc. - Exponential or factorial time complexity: The algorithm's running time or space requirements grow exponentially or factorially with the input size. Examples include exhaustive search algorithms or certain combinatorial problems.

The asymptotic complexity provides a high-level understanding of how an algorithm's performance scales with input size. It helps in comparing and choosing algorithms, identifying bottlenecks, and predicting their behavior for large inputs. However, it's important to note that asymptotic complexity analysis focuses on the worst-case scenario and may not accurately represent the actual performance of an algorithm in all situations. Other factors like average-case complexity, best-case complexity, and real-world considerations should also be taken into account when analyzing and selecting algorithms.

## Big O notation

Big O notation is a mathematical notation used in computer science to describe the asymptotic behavior of an algorithm. It provides a way to express the upper bound or worst-case complexity of an algorithm in terms of the input size.

In Big O notation, the letter "O" is followed by a function that represents the upper bound of the algorithm's growth rate. The notation is written as O(f(n)), where "f(n)" is a mathematical function of the input size "n". The function "f(n)" represents the maximum number of operations or amount of space used by the algorithm as a function of the input size.

The Big O notation is used to classify algorithms into different complexity classes based on their growth rates. It allows us to compare and analyze algorithms in terms of their efficiency and scalability.

Here are some common examples of Big O notation:

• O(1) - Constant time complexity: The algorithm's running time or space requirements remain constant regardless of the input size.

• O(log n) - Logarithmic time complexity: The algorithm's running time or space requirements increase logarithmically with the input size.

• O(n) - Linear time complexity: The algorithm's running time or space requirements grow linearly with the input size.

• O(n^2), O(n^3), etc. - Polynomial time complexity: The algorithm's running time or space requirements grow with a polynomial function of the input size.

• O(2^n), O(n!), etc. - Exponential or factorial time complexity: The algorithm's running time or space requirements grow exponentially or factorially with the input size.

The Big O notation provides a way to describe the scalability of an algorithm and helps in comparing and selecting algorithms based on their efficiency. It focuses on the worst-case scenario and provides an upper bound on the algorithm's performance. However, it's important to consider other factors like average-case complexity and real-world considerations when analyzing and choosing algorithms.

## Complexity classes

 O Complexity type O(1) constant O(log(n)) logarithmic O(n) linear O(n×log(n)) quasi-linear O(n2) quadratic O(n3) cubic O(2n) exponential O(n!) factorial

## Worst case complexity

The worst-case complexity of an algorithm refers to the maximum amount of time or space that the algorithm requires to run, given the worst-case input. It represents the upper bound on the algorithm's performance in the most unfavorable scenario.

In Big O notation, the worst-case complexity is denoted by the notation O(f(n)), where "f(n)" represents a mathematical function that describes the growth rate of the algorithm as a function of the input size "n". The worst-case complexity provides an upper bound on the running time or space requirements of the algorithm, ensuring that it will not perform worse than this bound for any input of size "n" or larger.

The worst-case complexity is particularly useful when analyzing and comparing algorithms because it guarantees that the algorithm will not exceed a certain level of performance, regardless of the input data. It helps in understanding the scalability and efficiency of the algorithm in the worst possible scenario.

When evaluating algorithms, it's important to consider both the worst-case complexity and other factors such as average-case complexity, best-case complexity, and real-world considerations. The worst-case complexity provides a baseline measure to assess the efficiency and scalability of an algorithm, but it may not reflect the actual performance in typical or average scenarios.