144. Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new LinkedList<Integer>();
        if(root!=null)
		preOrder(root, result);
		return result;
    }
    public static void preOrder(TreeNode node, List<Integer> list)
	{
		list.add(node.val);
		if (node.left!=null) {
			preOrder(node.left, list);
		}
		if (node.right!=null) {
			preOrder(node.right, list);
		}
	}
}