二叉树前序遍历Binary Tree Preorder Traversal (Java)

public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}

public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> returnList = new ArrayList<Integer>();

if(root == null)
return returnList;

Stack<TreeNode> stack = new Stack<TreeNode>();
stack.push(root);

while(!stack.empty()){
TreeNode n = stack.pop();
returnList.add(n.val);

if(n.right != null){
stack.push(n.right);
}
if(n.left != null){
stack.push(n.left);
}

}
return returnList;
}
}