589. N-ary Tree Preorder Traversal

leetcode.com/problems/n-ary-tree-preorder-traversal/

N-ary Tree Preorder Traversal - LeetCode

/*
    자손이 N개인 트리의 pre-order
*/

class Solution {
    
    public List<Integer> preorder(Node root) {
        List<Integer> ret = new ArrayList<Integer>();
        traverse(root, ret);
        return ret;
    }
    
    public void traverse(Node root, List<Integer> ret) {
        if(root == null) {
            return;
        }
        ret.add(root.val);
        for(Node child : root.children) {
            traverse(child, ret);
        }
    }
 
 
 	// iterative
    public List<Integer> preorder(Node root) {
        Stack<Node> stk = new Stack<Node>();
        List<Integer> preorder = new LinkedList<Integer>();
        stk.push(root);
        
        if(root == null) {
            return preorder;
        }
        
        while(!stk.isEmpty()) {
            Node curr = stk.pop();
            preorder.add(curr.val);
            for(int i = curr.children.size() - 1; i >= 0; i--) {
                stk.push(curr.children.get(i));
            }
        }
        return preorder;
	}
 
 
}

leetcode.com/problems/n-ary-tree-preorder-traversal/discuss/565696/Iterative-and-Recursive-Java-Solutions