I'm trying to implement the Union-Find algorithm, but all of the implementations i've looked up use integers. I need to implement the algorithm so that I can call the union() and the connected() methods in this way: union(Vertex v, Vertex, w) - connected(Vertex v, Vertex w)
I've tried adapting my algorithm for it to work with vertices, but I don't know how to replace the parent and rank atributes to make it work. Please help :(
public class UF {
private int[] parent; // parent[i] = parent of i
private byte[] rank; // rank[i] = rank of subtree rooted at i (never more than 31)
private int count; // number of components
/**
* Initializes an empty union–find data structure with {@code n} sites
* {@code 0} through {@code n-1}. Each site is initially in its own
* component.
*
* @param n the number of sites
* @throws IllegalArgumentException if {@code n < 0}
*/
public UF(int n) {
if (n < 0) throw new IllegalArgumentException();
count = n;
parent = new int[n];
rank = new byte[n];
for (int i = 0; i < n; i++) {
parent[i] = i;
rank[i] = 0;
}
}
/**
* Returns the component identifier for the component containing site {@code p}.
*
* @param p the integer representing one site
* @return the component identifier for the component containing site {@code p}
* @throws IllegalArgumentException unless {@code 0 <= p < n}
*/
public int find(int p) {
validate(p);
while (p != parent[p]) {
parent[p] = parent[parent[p]]; // path compression by halving
p = parent[p];
}
return p;
}
/**
* Returns the number of components.
*
* @return the number of components (between {@code 1} and {@code n})
*/
public int count() {
return count;
}
/**
* Returns true if the the two sites are in the same component.
*
* @param p the integer representing one site
* @param q the integer representing the other site
* @return {@code true} if the two sites {@code p} and {@code q} are in the same component;
* {@code false} otherwise
* @throws IllegalArgumentException unless
* both {@code 0 <= p < n} and {@code 0 <= q < n}
*/
public boolean connected(int p, int q) {
return find(p) == find(q);
}
/**
* Merges the component containing site {@code p} with the
* the component containing site {@code q}.
*
* @param p the integer representing one site
* @param q the integer representing the other site
* @throws IllegalArgumentException unless
* both {@code 0 <= p < n} and {@code 0 <= q < n}
*/
public void union(int p, int q) {
int rootP = find(p);
int rootQ = find(q);
if (rootP == rootQ) return;
// make root of smaller rank point to root of larger rank
if (rank[rootP] < rank[rootQ]) parent[rootP] = rootQ;
else if (rank[rootP] > rank[rootQ]) parent[rootQ] = rootP;
else {
parent[rootQ] = rootP;
rank[rootP]++;
}
count--;
}
// validate that p is a valid index
private void validate(int p) {
int n = parent.length;
if (p < 0 || p >= n) {
throw new IllegalArgumentException("index " + p + " is not between 0 and " + (n-1));
}
}
}
In the standard algorithm each vertex is given an int
id which represents it's place in an array. So this means parent[0]
contains the id of the parent of vertex 0 and so.
Really you can consider arrays to be just a very efficient map from int
to something else. If you replace the int
with a more complex type then you need to start using a Map
rather than an array.
So if you want to use a class called Vertex
to represent vertices then you need to declare parents and ranks differently:
Map<Vertex,Vertex> parent = new HashMap<>();
Map<Vertex,Rank> rank = new HashMap<>();
You could replace Rank
with Byte
if you want to stick to the current scheme - though it's probably better encapsulation to use a class.
You'll then end up with code that looks something like:
while (!vertex.equals(parent.get(vertex))) {
parent.put(vertex, parent.get(parent.get(vertex)));
vertex = parent.get(vertex);
}
return vertex;
One thing to be aware of is that if you are going to use Vertex
as the key of a map (as I've recommended) then you must implement equals
and hashCode
methods.
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