feat: add Disjoint Set Data Structure using Java

Data Structures/DisjointSet/DisjointSet.java

@@ -0,0 +1,42 @@
+public class DisjointSet {
+        private int[] parent;
+
+        public DisjointSet(int n) {
+            parent = new int[n];
+            for (var i = 0; i < n; i++) {
+                parent[i] = i;
+            }
+        }
+
+        public int find(int x) {
+            if (x == parent[x]) {
+                return x;
+            }
+            // compress the paths
+            return parent[x] = find(parent[x]);
+        }
+
+        public void union(int x, int y)  {
+            var px = find(x);
+            var py = find(y);
+            if (px != py) {
+                parent[px] = py;
+            }
+        }
+        // number of groups
+        public int size() {
+            int ans = 0;
+            for (int i = 0; i < parent.length; ++ i) {
+                if (i == parent[i]) ans ++;
+            }
+            return ans;
+        }
+
+        public static void main(String[] args) {
+            var ds = new DisjointSet(5);
+            System.out.println(ds.find(3));
+
+            ds.union(3, 4);
+            System.out.println(ds.find(3)); // after join, 3's parent is 4.
+        }
+    }