n叉树的先序遍历，并给出测试例。用java语言编写

以下是n叉树的先序遍历的Java代码实现：

class TreeNode {
    int val;
    List<TreeNode> children;
    TreeNode(int val) {
        this.val = val;
        this.children = new ArrayList<>();
    }
}

public class NaryTreePreorderTraversal {
    public List<Integer> preorder(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode curr = stack.pop();
            res.add(curr.val);
            for (int i = curr.children.size() - 1; i >= 0; i--) {
                stack.push(curr.children.get(i));
            }
        }
        return res;
    }
}

以下是一个测试例：

public static void main(String[] args) {
    TreeNode root = new TreeNode(1);
    TreeNode node2 = new TreeNode(2);
    TreeNode node3 = new TreeNode(3);
    TreeNode node4 = new TreeNode(4);
    TreeNode node5 = new TreeNode(5);
    TreeNode node6 = new TreeNode(6);
    root.children.add(node2);
    root.children.add(node3);
    root.children.add(node4);
    node3.children.add(node5);
    node3.children.add(node6);
    NaryTreePreorderTraversal solution = new NaryTreePreorderTraversal();
    List<Integer> res = solution.preorder(root);
    System.out.println(res); // Output: [1, 2, 3, 5, 6, 4]
}

在这个测试例中，我们创建了一个n叉树，并对其进行先序遍历，结果为[1, 2, 3, 5, 6, 4]。