Given a binary tree, return the preorder traversal of its nodes' values.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

Method 1. divide and conquer
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();

        if(root == null) return res;

        List<Integer> left = preorderTraversal(root.left);
        List<Integer> right = preorderTraversal(root.right);

        res.add(root.val);
        res.addAll(left);
        res.addAll(right);

        return res;
    }
}


Method 2. use iteration with stack
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();

        if(root == null) return res;

        stack.push(root);
        while(!stack.empty()) {
            TreeNode curNode = stack.pop();
            res.add(curNode.val);
            if(curNode.right != null) stack.push(curNode.right);
            if(curNode.left != null) stack.push(curNode.left);
        }

        return res;
    }
}