java: add «590. N-ary Tree Postorder Traversal»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/Node.java

@@ -0,0 +1,18 @@
+import java.util.List;
+
+public class Node {
+  public int val;
+  public List<Node> children;
+
+  public Node() {}
+
+  public Node(int _val) {
+    val = _val;
+  }
+
+  public Node(int _val, List<Node> _children) {
+    val = _val;
+    children = _children;
+  }
+}
+;

java/n-ary-tree-postorder-traversal.java

@@ -0,0 +1,21 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+  private ArrayList<Integer> postorder(Node node, ArrayList<Integer> l) {
+    if (node == null) {
+      return l;
+    }
+
+    for (var child : node.children) {
+      postorder(child, l);
+    }
+    l.add(node.val);
+
+    return l;
+  }
+
+  public List<Integer> postorder(Node root) {
+    return postorder(root, new ArrayList<Integer>());
+  }
+}