Binary Tree Preorder Traversal @leetcode

PreorderTraversal.java

package leetcode.tree;

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

import leetcode.tree.InorderTraversal.TreeNode;

public class PreorderTraversal {

	public ArrayList<Integer> preorderTraversal(TreeNode root) {
        TreeNode node = root;
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> arrayList = new ArrayList<Integer>();
        
        while (node != null || stack.size() > 0) {
            while (node != null) {
                arrayList.add(node.val);
                stack.push(node);
                node = node.left;
            }
            
            if (stack.size() > 0) {
                node = stack.pop();
                node = node.right;
            }
        }
        
        return arrayList;
    }
}