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

For example:
Given binary tree {1,#,2,3},

   1
         2
    /
   3

return [1,2,3].

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
   public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> nodeStack=new Stack<TreeNode>();
        List<Integer> list=new ArrayList<Integer>();
        TreeNode node=root;
        while(!nodeStack.isEmpty() || node!=null){
            if(node!=null){
                list.add(node.val);
                if(node.right!=null){
                    nodeStack.add(node.right);
                }
                node=node.left;
            }
            else{
            	node=nodeStack.peek();
            	nodeStack.pop();
            }
        }
        return list;
    }
}