多叉树的前序遍历

LeetCode原题：(https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/comments/)

class Solution {
    public List<Integer> preorder(Node root) {
        if(root == null) return new ArrayList<>();
        Stack<Node> stack = new Stack<>();
        List<Integer> result = new ArrayList<>();
        stack.push(root);
        while(!stack.isEmpty()){
            Node node = stack.pop();
            result.add(node.val);
            if (node.children != null && !node.children.isEmpty()) {
                // 应该倒过来将子节点放入，因为在上文中使用的数据结构是栈
                for (int i = node.children.size() - 1; i >= 0; i--) {
                    stack.push(node.children.get(i));
                }
            }
          } 
        return result;
    }
}