145. Binary Tree Postorder Traversal

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

class Solution {
    List<Integer> result = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        helper(root);
        return result;
    }

    private void helper(TreeNode root) {
        if (root == null)
            return;
        if (root.left != null)
            helper(root.left);
        if (root.right != null)
            helper(root.right);
        result.add(root.val);
    }
}