Binary Tree Postorder Traversal

public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> rst = new ArrayList<Integer>();
        if(root == null) return rst;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty())
        {
            TreeNode node = stack.pop();
            rst.add(0,node.val);
            if(node.left != null)
                stack.push(node.left);
            if(node.right != null)
                stack.push(node.right);
        }
        return rst;
    }
}