What Is Collision in Data Structure Algorithm?


Scott Campbell

A collision in data structure algorithms occurs when two or more elements are mapped to the same location in a data structure, such as a hash table or a hash map. This can happen due to the limited range of indices or keys in the data structure, which results in different elements being hashed to the same index.

Understanding Collision

When we insert an element into a data structure that uses hashing for indexing, such as a hash table, the element’s key is transformed into an index using a hash function. The resulting index determines where the element will be stored in the data structure.

In an ideal scenario, each element would have a unique index and there would be no collisions. However, due to the limited number of available indices and the potentially large number of elements being stored, collisions can occur.

Types of Collision Resolution

There are various strategies for handling collisions in data structure algorithms:

  • 1. Separate Chaining: This strategy involves maintaining a linked list at each index of the data structure. When a collision occurs, the new element is simply appended to the linked list at that index.

    Retrieval of elements requires traversing through the linked list starting from the hashed index.

  • 2. Open Addressing: In this strategy, when a collision occurs, alternative locations within the data structure are probed until an empty slot is found. There are different methods for probing, such as linear probing (checking consecutive indices), quadratic probing (checking indices with quadratic increments), or double hashing (using a secondary hash function).

Handling Collisions – Example

To illustrate how collisions are handled in practice, let’s consider a simple example using separate chaining:

Suppose we have a hash table with 10 indices and the following elements:

  • Element A with key 3
  • Element B with key 13
  • Element C with key 23

Assuming our hash function simply takes the modulus of the key by the number of indices, we can calculate the following hashed indices:

  • Element A: Hashed index = 3 % 10 = 3
  • Element B: Hashed index = 13 % 10 = 3
  • Element C: Hashed index = 23 % 10 = 3

Note that all three elements are hashed to index 3, resulting in a collision. To handle this, we can create a linked list at index 3 and append the elements accordingly:

  • Index3:
    • A -> B -> C

This way, we can store multiple elements at the same index and retrieve them by traversing through the linked list.

The Impact of Collisions on Performance

Collisions can have an impact on the performance of data structure algorithms. When collisions occur frequently, it may result in longer retrieval times due to having to traverse through linked lists or perform additional probing operations.

The efficiency of collision resolution strategies also plays a role in performance. While separate chaining provides a simple solution to collisions, it requires additional memory for storing linked lists. On the other hand, open addressing strategies can lead to clustering and increased probing operations.


Collisions are an inevitable occurrence in data structure algorithms when multiple elements are mapped to the same index. Understanding collision resolution strategies and their impact on performance is crucial for designing efficient data structures. By implementing appropriate collision resolution techniques, we can minimize the negative effects of collisions and optimize the performance of our algorithms.

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