Binary Tree Preorder Traversal

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

Have you met this question in a real interview? 

Yes

Example

Given binary tree {1,#,2,3}:

1
 \
  2
 /
3

return [1,2,3].

Challenge

Can you do it without recursion?

public class Solution {

    /**

     * @param root: The root of binary tree.

     * @return: Preorder in ArrayList which contains node values.

     */

    public ArrayList<Integer> preorderTraversal(TreeNode root) {

        // write your code here

        ArrayList<Integer> result = new ArrayList<Integer>();

        Stack<TreeNode> stack = new Stack<TreeNode>();

        if(root != null)

        stack.push(root);

        while(!stack.isEmpty()){

            root = stack.pop();

            result.add(root.val);

            if(root.right != null) stack.push(root.right);

            if(root.left != null) stack.push(root.left);

        }

        return result;

    }

}

recursion:

public class Solution {

    /**

     * @param root: The root of binary tree.

     * @return: Preorder in ArrayList which contains node values.

     */

    public ArrayList<Integer> preorderTraversal(TreeNode root) {

        // write your code here

        ArrayList<Integer> result = new ArrayList<Integer>();

        recursion(root, result);

        return result;

    }

    private void recursion(TreeNode root,  ArrayList<Integer> result){

        if(root == null) return;

        result.add(root.val);

        recursion(root.left, result);

        recursion(root.right, result);

    }

}