Java实现 LeetCode 590 N叉树的后序遍历（遍历树，迭代法）

590. N叉树的后序遍历

给定一个 N 叉树，返回其节点值的后序遍历。

例如，给定一个 3叉树 :

返回其后序遍历: [5,6,3,2,4,1].

说明: 递归法很简单，你可以使用迭代法完成此题吗?

/*
// 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 {
     public List<Integer> postorder(Node root) {
            LinkedList<Integer> output = new LinkedList<>();
            if (root == null) {
                return output;
            }
            Deque<Node> stack = new LinkedList<>();
            stack.add(root);
            while (!stack.isEmpty()) {
                Node curNode = stack.pollLast();
                output.addFirst(curNode.val);
                if (curNode.children != null && !curNode.children.isEmpty()) {
                    stack.addAll(curNode.children);
                }
            }
            return output;
    }
}