leetcode PreorderTravel

import utils.ListNode;
import utils.TreeNode;

import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

public class PreorderTravel {
    /*
        Input: [1,null,2,3]

           1
            \
             2
            /
           3

        Output: [1,2,3]
        better not recurritive but iterative
     */
    public static void main(String[] args){

    }
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        List<Integer> ans = new LinkedList<>();

        if (root != null){
            s.push(root);
        }
        while(!s.isEmpty()) {
            root = s.pop();
            ans.add(root.val);
            TreeNode left = root.left;
            TreeNode right = root.right;
            if (right != null) {
                s.push(right);
            }
            if (left != null){
                s.push(left);
            }

        }
        return ans;



    }
}