diff --git a/src/main/scala/io/github/acl4s/Dsu.scala b/src/main/scala/io/github/acl4s/Dsu.scala
new file mode 100644
index 0000000..ef120bb
--- /dev/null
+++ b/src/main/scala/io/github/acl4s/Dsu.scala
@@ -0,0 +1,80 @@
+package io.github.acl4s
+
+import scala.collection.mutable
+
+/**
+ * Implement (union by size) + (path compression)
+ * Reference:
+ * Zvi Galil and Giuseppe F. Italiano,
+ * Data structures and algorithms for disjoint set union problems
+ *
+ * @param n
+ */
+case class Dsu(n: Int) {
+
+  /**
+   * root node: -1 * component size
+   * otherwise: parent
+   */
+  private val parentOrSize: Array[Int] = Array.fill(n)(-1)
+
+  def merge(a: Int, b: Int): Int = {
+    assert(0 <= a && a < n)
+    assert(0 <= b && b < n)
+    var x = leader(a)
+    var y = leader(b)
+    if (x == y) { return x }
+    if (-parentOrSize(x) < -parentOrSize(y)) {
+      // std::swap(x, y);
+      val z = x
+      x = y
+      y = z
+    }
+    parentOrSize(x) += parentOrSize(y)
+    parentOrSize(y) = x
+    x
+  }
+
+  def same(a: Int, b: Int): Boolean = {
+    assert(0 <= a && a < n)
+    assert(0 <= b && b < n)
+    leader(a) == leader(b)
+  }
+
+  def leader(a: Int): Int = {
+    assert(0 <= a && a < n)
+    if (parentOrSize(a) < 0) {
+      a
+    } else {
+      parentOrSize(a) = leader(parentOrSize(a))
+      parentOrSize(a)
+    }
+  }
+
+  def size(a: Int): Int = {
+    assert(0 <= a && a < n)
+    -parentOrSize(leader(a))
+  }
+
+  def groups(): Seq[Seq[Int]] = {
+    val leader_buf = new Array[Int](n)
+    val group_size = new Array[Int](n)
+    (0 until n).foreach(i => {
+      leader_buf(i) = leader(i)
+      group_size(leader_buf(i)) += 1
+    })
+
+    val result = new Array[mutable.ArrayBuffer[Int]](n)
+    (0 until n).foreach(i => {
+      result(i) = new mutable.ArrayBuffer(group_size(i))
+    })
+    (0 until n).foreach(i => {
+      result(leader_buf(i)) += i
+    })
+
+    result.collect {
+      case buf if buf.nonEmpty => buf.toSeq
+    }.toSeq
+  }
+
+}
diff --git a/src/test/scala/io/github/acl4s/DsuSuite.scala b/src/test/scala/io/github/acl4s/DsuSuite.scala
new file mode 100644
index 0000000..14d234b
--- /dev/null
+++ b/src/test/scala/io/github/acl4s/DsuSuite.scala
@@ -0,0 +1,68 @@
+package io.github.acl4s
+
+class DsuSuite extends munit.FunSuite {
+
+  /**
+   * @see https://atcoder.jp/contests/practice2/tasks/practice2_a
+   */
+  test("AtCoder Library Practice Contest A - Disjoint Set Union") {
+    val uf = Dsu(4)
+
+    assertEquals(uf.same(0, 1), false)
+
+    uf.merge(0, 1)
+    uf.merge(2, 3)
+
+    assertEquals(uf.same(0, 1), true)
+    assertEquals(uf.same(1, 2), false)
+    assertEquals(uf.groups(), Seq(Seq(0, 1), Seq(2, 3)))
+
+    uf.merge(0, 2)
+
+    assertEquals(uf.same(1, 3), true)
+    assertEquals(uf.groups(), Seq(Seq(0, 1, 2, 3)))
+  }
+
+  test("zero") {
+    val uf = Dsu(0)
+
+    assertEquals(uf.groups(), Seq())
+  }
+
+  test("simple") {
+    val uf = Dsu(2)
+
+    assertEquals(uf.same(0, 1), false)
+
+    val x = uf.merge(0, 1)
+    assertEquals(uf.leader(0), x)
+    assertEquals(uf.leader(1), x)
+    assertEquals(uf.same(0, 1), true)
+    assertEquals(uf.size(0), 2)
+  }
+
+  test("line") {
+    val n = 500_000
+    val uf = Dsu(n)
+
+    (0 until n - 1).foreach(i => {
+      uf.merge(i, i + 1)
+    })
+
+    assertEquals(uf.size(0), n)
+    assertEquals(uf.groups().size, 1)
+  }
+
+  test("line reverse") {
+    val n = 500_000
+    val uf = Dsu(n)
+
+    (0 until n - 1).reverse.foreach(i => {
+      uf.merge(i, i + 1)
+    })
+
+    assertEquals(uf.size(0), n)
+    assertEquals(uf.groups().size, 1)
+  }
+
+}