[leetCode]590. N叉树的后序遍历

题目

https://leetcode-cn.com/problems/n-ary-tree-postorder-traversal/

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].

递归

class Solution {
    List<Integer> ans = new ArrayList();

    public List<Integer> postorder(Node root) {
        traversal(root);
        return ans;
    }

    private void traversal(Node node) {
        if (node == null)
            return;
        for (Node c : node.children) {
            traversal(c);
        }
        ans.add(node.val);
    }
}

迭代

class Solution {
    List<Integer> ans = new ArrayList();

    public List<Integer> postorder(Node root) {
        if (root == null) return ans;
         LinkedList<Node> stack = new LinkedList<>();
         stack.push(root);
         while (!stack.isEmpty()) {
            Node node = stack.pop();
            if (node != null) {
                stack.push(node);
                stack.push(null);
                for (int i = node.children.size() - 1; i >= 0; i--) { // 左后进所以左先出
                if (node.children.get(i) != null) 
                    stack.push(node.children.get(i) );
                }
            
            } else {
                Node cur = stack.pop();
                ans.add(cur.val);
            }
         }
        return ans;
    }

}