[Leetcode] Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

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

比较简单的链表题，其实也是模拟题，模拟加法运算的过程，只需要留意最后可能多出来的进位不要忘记加上去就好了。

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode temp = dummy;
        int carry = 0, sum = 0;
        
        while(l1 != null && l2 != null) {
            sum = l1.val + l2.val + carry;
            carry = sum / 10;
            ListNode node = new ListNode(sum % 10);
            temp.next = node;
            temp = temp.next;
            l1 = l1.next; l2 = l2.next;
        }
        while(l1 != null) { // only one of these will be excuted
            sum = l1.val + carry;
            carry = sum / 10;
            ListNode node = new ListNode(sum % 10);
            temp.next = node;
            temp = temp.next;
            l1 = l1.next;
        }
        while(l2 != null) {
            sum = l2.val + carry;
            carry = sum / 10;
            ListNode node = new ListNode(sum % 10);
            temp.next = node;
            temp = temp.next;
            l2 = l2.next;
        }
        if(carry > 0) { // important here
            ListNode node = new ListNode(carry);
            temp.next = node;
        }
        
        return dummy.next;
    }