【leetcode】590. N叉树的后序遍历（利用两个栈实现）

/*
// 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) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        if(root == null) return list;
        Node node = root;
        Stack<Node> stack1 = new Stack<>();
        Stack<Node> stack2 = new Stack<>();
        stack1.push(root);
        while(!stack1.isEmpty()) {
            node = stack1.pop();
            stack2.add(node);
            if(node.children != null) {
                List<Node> c = node.children;
                for(Node n:c) {
                    stack1.push(n);
                }
            } 
        }
        while(!stack2.isEmpty()) {
            list.add(stack2.pop().val);
        }
        return list;
    }


    public List<Integer> postorder2(Node root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        Node node = null;
        Node temp = null;
        Stack<Node> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            temp = stack.peek();
            if (temp.children==null || (node!=null && temp.children.contains(node))) {
                node = stack.pop();
                list.add(node.val);
            } else {
                if (temp.children!=null) {
                    for (int i = temp.children.size()-1; i >= 0 ; i--) {
                        stack.push(temp.children.get(i));
                    }
                }
            }
        }
        return list;
    }
}