problems: add n-ary tree preorder traversal

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

problems/n-ary-tree-preorder-traversal.cpp

@@ -0,0 +1,43 @@
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    vector<Node*> children;
+
+    Node() { }
+
+    Node(int _val)
+    {
+        val = _val;
+    }
+
+    Node(int _val, vector<Node*> _children)
+    {
+        val = _val;
+        children = _children;
+    }
+};
+*/
+
+class Solution {
+    void preorder(vector<int>& traversal, Node* root)
+    {
+        if (root == nullptr) {
+            return;
+        }
+
+        traversal.push_back(root->val);
+        for (auto child : root->children) {
+            preorder(traversal, child);
+        }
+    }
+
+public:
+    vector<int> preorder(Node* root)
+    {
+        vector<int> result;
+        preorder(result, root);
+        return result;
+    }
+};