129. sum root to leaf numbers

class  {
    int sum;
    public int sumNumbers(TreeNode root) {
        sum = 0;
        dfs(root, 0);
        return sum;
    }
    
    public void dfs(TreeNode node, int cur) {
        if (node == null) return;
        cur = cur * 10 + node.val;
        if (node.left == null && node.right == null) {
            sum += cur;
            return;
        }
        if (node.left != null) dfs(node.left, cur);
        if (node.right != null) dfs(node.right, cur);
    }

    public int iterative(TreeNode root) {
        if (root == null) return 0;
        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> sum = new Stack<>(); 
        stack.push(root);
        sum.push(root.val);
        int res = 0;
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            int cur = sum.pop();
            if (node.left == null && node.right == null) {
                res += cur;
            }
            if (node.right != null) {
                stack.push(node.right);
                sum.push(cur * 10 + node.right.val);
            }
            if (node.left != null) {
                stack.push(node.left);
                sum.push(cur * 10 + node.left.val);
            }
        }
        return res;
    }
}