題目:
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) + (4 -> 6 -> 4)
Output: 6 -> 0 -> 8
題解：
這道題就是給的加數是倒着給的，你返回的結果也是倒着寫的，所以進位也反着進就好。
維護一個carry，加數大於9時候carry為1，也要注意兩個數相加如果大於8的話，要寫取模后的值。
代碼如下：
1
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
2 if(l1== null)
3 return l2;
4 if(l2== null)
5 return l1;
6
7 int carry = 0;
8 ListNode newhead = new ListNode(-1);
9 ListNode l3 = newhead;
10
11 while(l1!= null || l2!= null){
12 if(l1!= null){
13 carry += l1.val;
14 l1 = l1.next;
15 }
16 if(l2!= null){
17 carry += l2.val;
18 l2 = l2.next;
19 }
20
21 l3.next = new ListNode(carry%10);
22 carry = carry/10;
23 l3 = l3.next;
24 }
25
26 if(carry == 1)
27 l3.next= new ListNode(1);
28 return newhead.next;
29 }
2 if(l1== null)
3 return l2;
4 if(l2== null)
5 return l1;
6
7 int carry = 0;
8 ListNode newhead = new ListNode(-1);
9 ListNode l3 = newhead;
10
11 while(l1!= null || l2!= null){
12 if(l1!= null){
13 carry += l1.val;
14 l1 = l1.next;
15 }
16 if(l2!= null){
17 carry += l2.val;
18 l2 = l2.next;
19 }
20
21 l3.next = new ListNode(carry%10);
22 carry = carry/10;
23 l3 = l3.next;
24 }
25
26 if(carry == 1)
27 l3.next= new ListNode(1);
28 return newhead.next;
29 }
Reference: http://www.programcreek.com/2012/12/add-two-numbers/