Add Union Find (fixes #2692) (#2693)

Searches/UnionFind.java

@@ -0,0 +1,77 @@
+package Searches;
+
+import java.util.*;
+
+public class UnionFind {
+    private int[] p;
+    private int[] r;
+
+    public UnionFind(int n) {
+        p = new int[n];
+        r = new int[n];
+ 
+        for (int i = 0; i < n; i++) {
+            p[i] = i;
+        }
+    }
+ 
+    public int find(int i) {
+        int parent = p[i];
+        
+        if (i == parent) return i;
+
+        return p[i] = find(parent);
+    }
+
+    public void union(int x, int y) {
+        int r0 = find(x);
+        int r1 = find(y);
+ 
+        if (r1 == r0) return;
+   
+        if (r[r0] > r[r1]) {
+            p[r1] = r0;
+        } else if (r[r1] > r[r0]) {
+            p[r0] = r1;
+        } else {
+            p[r1] = r0;
+            r[r0]++;
+        }
+    }
+
+    public int count() {
+        List parents = new ArrayList();
+        for(int i = 0; i < p.length; i++) {
+            if(!parents.contains(find(i))) {
+                parents.add(find(i));
+            }
+        }
+        return parents.size();
+    }
+
+    public String toString() {
+        return "p " + Arrays.toString(p) + " r " + Arrays.toString(r) + "\n";
+    }
+  
+    // Tests
+    public static void main(String[] args) {
+        UnionFind uf = new UnionFind(5);
+        System.out.println("init /w 5 (should print 'p [0, 1, 2, 3, 4] r [0, 0, 0, 0, 0]'):");
+        System.out.println(uf);
+        
+        uf.union(1,2);
+        System.out.println("union 1 2 (should print 'p [0, 1, 1, 3, 4] r [0, 1, 0, 0, 0]'):");
+        System.out.println(uf);
+
+        uf.union(3,4);
+        System.out.println("union 3 4 (should print 'p [0, 1, 1, 3, 3] r [0, 1, 0, 1, 0]'):");
+        System.out.println(uf);
+
+        uf.find(4);
+        System.out.println("find 4 (should print 'p [0, 1, 1, 3, 3] r [0, 1, 0, 1, 0]'):");
+        System.out.println(uf);
+
+        System.out.println("count (should print '3'):");
+        System.out.println(uf.count());
+    }
+}