两数相加

public ListNode addTwoNumbers(ListNode l1,ListNode l2){
        Stack<Integer> stack1 = buildStack(l1);
        Stack<Integer> stack2 = buildStack(l2);
        ListNode head = new ListNode();
        int carry = 0;
        while(!stack1.isEmpty() || !stack2.isEmpty() || carry!=0){
            int x = stack1.isEmpty()?0:stack1.pop();
            int y = stack2.isEmpty()?0:stack2.pop();
            int sum = x + y + carry;
            ListNode node  = new ListNode();
            node.next = head.next;
            head.next = node;
            carry = sum/10;
        }
        return head.next;
    }
    
    private Stack<Integer> buildStack(ListNode l){
        Stack<Integer> stack = new Stack<>();
        while(l!=null){
            stack.push(l.val);
            l = l.next;
        }
        return stack;
    }