problems(java): add „429. N-ary Tree Level Order Traversal“

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/n-ary-tree-level-order-traversal.java

@@ -0,0 +1,41 @@
+/*
+// Definition for a Node.
+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;
+    }
+};
+*/
+
+class Solution {
+    private void levelOrder(ArrayList<List<Integer>> traversal, Node root, int level) {
+        if (root == null) {
+            return;
+        }
+
+        if (level >= traversal.size()) {
+            traversal.add(new ArrayList<Integer>());
+        }
+        traversal.get(level).add(root.val);
+
+        for (var child : root.children) {
+            levelOrder(traversal, child, level + 1);
+        }
+    }
+
+    public List<List<Integer>> levelOrder(Node root) {
+        ArrayList<List<Integer>> result = new ArrayList<>();
+        levelOrder(result, root, 0);
+        return result;
+    }
+}