What Is Amortized Complexity in Data Structure?


Heather Bennett

Amortized complexity is a concept in data structures and algorithms that measures the average time or space required to perform a sequence of operations. It provides a way to analyze the efficiency of an algorithm over multiple operations rather than just considering individual operations in isolation. This can be particularly useful when dealing with data structures that have occasional expensive operations but are efficient overall.

Understanding Amortized Complexity

Before diving into amortized complexity, let’s first understand what we mean by the term “complexity” in the context of data structures and algorithms. Complexity refers to the amount of time or space required by an algorithm to solve a problem, often measured in terms of big O notation.

When analyzing algorithms, we commonly encounter three types of complexity: worst-case, best-case, and average-case complexity. Worst-case complexity measures the maximum amount of resources an algorithm requires for any input size.

Best-case complexity represents the minimum amount of resources needed for any input size. Average-case complexity calculates the expected resources required on average for various input sizes.


The concept of amortization allows us to analyze data structures by distributing the cost of expensive operations across multiple cheaper operations. In other words, it takes into account both costly and inexpensive operations together to provide an overall understanding of efficiency.

To explain this further, let’s take an example using a dynamic array data structure, also known as an ArrayList. An ArrayList is similar to an array but can dynamically resize itself when needed.

  • Adding Elements: When adding elements to an ArrayList, if there is sufficient space available at its current capacity, adding an element has a time complexity of O(1), as it simply involves updating the last index and increasing its size by one.
  • Resizing: However, when the ArrayList reaches its capacity and needs to resize, it creates a new array with double the size of the previous one and copies all elements from the old array to the new one. This resizing operation has a time complexity of O(n), where n is the number of elements in the ArrayList.

Now, if we analyze individual operations, adding an element would have a complexity of O(1), while resizing would have a complexity of O(n). However, if we consider a sequence of n insertions, the overall cost can be amortized to O(1) per insertion.

Types of Amortization

There are two common types of amortization: aggregate analysis and accounting method.

Aggregate Analysis: Aggregate analysis calculates the total cost for a sequence of operations and then divides it by the number of operations to determine amortized complexity. In our ArrayList example, we divide the overall time taken for n insertions by n to get an average time complexity per insertion.

Accounting Method: The accounting method assigns different costs to different operations based on their actual expenses. For example, we can assign each insertion operation in our ArrayList example a cost of 2 units – 1 unit for adding an element and 1 unit for potential resizing.

The extra unit assigned during cheap operations is stored as “credit” that offsets future expensive operations. This way, costly operations are “paid for” by previously accumulated credits, resulting in amortized efficiency.

In Conclusion

Amortized complexity provides a valuable way to analyze data structures and algorithms over multiple operations rather than focusing on individual operations alone. It allows us to understand how efficient an algorithm is on average and provides insights into how expensive operations can be balanced by cheaper ones.

By using amortized complexity analysis, we can make informed decisions when choosing data structures and algorithms for our applications, considering both worst-case and average-case scenarios. It helps us design efficient systems that provide consistent performance even during periods of intense usage or when dealing with large datasets.

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