589. N叉树的前序遍历

深度优先搜索

class Solution {

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

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

迭代

class Solution {

    public List<Integer> preorder(Node root) {

        List<Integer> list = new LinkedList<>();
        Stack<Node> stack = new Stack<>();

        if (root == null){
            return list;
        }

        stack.push(root);

        while (!stack.isEmpty()){

            Node temp = stack.pop();
            list.add(temp.val);

            for (int i = temp.children.size() - 1; i >= 0; i--) {
                stack.push(temp.children.get(i));
            }
        }

        return list;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

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