LeetCode --- 589. N叉树的前序遍历

• LeetCode链接：589. N叉树的前序遍历

/*
// 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;
    }
};
*/

• 解法1：递归

class Solution {
    List<Integer> list = new LinkedList<>();
    public List<Integer> preorder(Node root) {
        if (root == null) return list;
        list.add(root.val);
        for (Node node : root.children) preorder(node);
        return list;
    }
}

• 解法2：非递归

class Solution {
    List<Integer> list = new LinkedList<>();
    public List<Integer> preorder(Node root) {
        List<Integer> list = new LinkedList<>();
        if (root == null) return list;
        Stack<Node> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            Node node = stack.pop();
            list.add(node.val);
            Collections.reverse(node.children);
            for (Node item : node.children) {
                stack.push(item);
            }
        }
        return list;
    }
}