Which Data Structure Is Used in Kruskal Algorithm?


Scott Campbell

When it comes to the Kruskal algorithm, a specific data structure plays a vital role in its implementation. This data structure is none other than the Disjoint Set, also known as the Union-Find data structure. It allows for efficient handling of connectivity between elements and is crucial in determining whether adding an edge to the Minimum Spanning Tree will create a cycle or not.

What is the Disjoint Set Data Structure?

The Disjoint Set data structure is designed to keep track of a collection of disjoint (non-overlapping) sets. It provides two key operations:

  • MakeSet(x): Creates a new set with element x.
  • Find(x): Returns the representative (root) of the set that contains element x.

To efficiently implement these operations, each set is represented by a tree-like structure, where each node points to its parent. The representative of a set can be found by following the parent pointers until reaching the root node, which does not have a parent.

Union-Find Operations

The Union-Find data structure also provides two important operations:

  • Union(x, y): Merges two sets containing elements x and y into one set.
  • Connected(x, y): Determines whether elements x and y belong to the same set or not.

To perform these operations efficiently, optimizations like rank-based union by rank and path compression are commonly used. These optimizations help keep the trees balanced and reduce path lengths during Find operations.

Role of Disjoint Set in Kruskal Algorithm

The Kruskal algorithm is a greedy algorithm used to find the Minimum Spanning Tree (MST) of a graph. It starts with an empty MST and iteratively adds edges in ascending order of their weights, as long as they don’t create a cycle.

The Disjoint Set data structure is utilized in the Kruskal algorithm to efficiently detect cycles. When adding an edge (u, v) to the MST, we check whether u and v already belong to the same set using the Connected operation of the Disjoint Set. If they do not, it means adding this edge will not create a cycle, so we merge their sets using the Union operation.

This process ensures that only non-cycle edges are added to the MST. By utilizing the Disjoint Set data structure, Kruskal’s algorithm achieves both efficiency and correctness in finding the Minimum Spanning Tree.


The Disjoint Set, or Union-Find, data structure is an essential component of Kruskal’s algorithm for finding Minimum Spanning Trees. Its efficient operations for checking connectivity and merging sets allow for fast detection of cycles and ensure that only non-cycle edges are added to the final MST. Understanding this data structure is crucial for implementing Kruskal’s algorithm effectively.

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