leetcode445. Add Two Numbers II

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

假设链表为1->2->3
则为其设置一个伪头：dummy->1->2->3， 并且记录当前需要交换的元素为cur
则每次转置如下：
dummy->1(cur)->2->3
dummy->2->1(cur)->3
dummy->3->2->1(cur)

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode rl1 = reverse(l1);
        ListNode rl2 = reverse(l2);
        ListNode result = new ListNode(0);
        int carry = 0;
        while(rl1 != null || rl2 != null || carry != 0) {
            int add = (rl1 == null ? 0 : rl1.val)
                    + (rl2 == null ? 0 : rl2.val)
                    + carry;
            carry = add / 10;
            ListNode tmp = new ListNode(add % 10);
            tmp.next = result.next;
            result.next = tmp;
            rl1 = rl1==null? rl1 : rl1.next;
            rl2 = rl2==null? rl2 : rl2.next;
        }
        return result.next;
    }
    
    public ListNode reverse(ListNode l) {
        ListNode dummy = new ListNode(0);
        dummy.next = l;
        ListNode cur = l;
        while(cur!= null && cur.next != null) {
            ListNode next = cur.next;
            cur.next = next.next;
            next.next = dummy.next;
            dummy.next = next;
        }
        return dummy.next;
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        while(l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        };
        while(l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }
        
        int carry = 0;
        ListNode result = new ListNode(0);
        while(!s1.isEmpty() || !s2.isEmpty() || carry != 0) {
            int add = (s1.isEmpty() ? 0 : s1.pop())
                    + (s2.isEmpty() ? 0 : s2.pop())
                    + carry;
            carry = add / 10;
            ListNode tmp = new ListNode(add % 10);
            tmp.next = result.next;
            result.next = tmp;
        }
        return result.next;
    }