589. N叉树的前序遍历

题解：
思路一：

递归

代码如下：

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ArrayList list = new ArrayList();
public List<Integer> preorder(Node root) {
    if (root == null) return list;
    
    Node node = root;
    list.add(node.val);

    for (int i = 0; i < root.children.size(); i++){
        
        preorder(node.children.get(i));
    }
    
    return list;
}
思路二：

迭代 , 跟二叉树前序遍历相同.

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public List<Integer> preorder(Node root) {
    if (root == null) return list;

    Stack<Node> stack = new Stack<>();
    stack.push(root);

    while (!stack.isEmpty()){

        Node node = stack.pop();
        list.add(node.val);

        for (int i = node.children.size() - 1; i >= 0; i--) {
            stack.push(node.children.get(i));
        }
    }

    return list;
}