144. Binary Tree Preorder Traversal |Time:O(n)|Space:O(1)

import java.util.List;
import java.util.ArrayList;
import java.util.Stack;

// Problem: https://leetcode.com/problems/binary-tree-preorder-traversal/
// Solution adapted from: https://www.youtube.com/watch?v=elQcrJrfObg
// Solution adapted from: https://www.programcreek.com/2012/12/leetcode-solution-for-binary-tree-preorder-traversal-in-java/

public class PreorderIterative {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public static List<Integer> preorderIterativeTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();

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

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();

            result.add(node.val);

            if(node.right != null) {
                stack.push(node.right);
            }

            if(node.left != null) {
                stack.push(node.left);
            }
        }

        return result;
    }

    public static void main(String args[]){

        TreeNode root = new TreeNode(10);
        root.left = new TreeNode(8);
        root.right = new TreeNode(2);
        root.left.left = new TreeNode(3);
        root.left.right = new TreeNode(5);
        root.left.right.left = new TreeNode(2);

        List<Integer> result = preorderIterativeTraversal(root);
        System.out.println(result.toString());
        // Expected output: 10 8 3 5 2 2
    }
}