2.两数相加

 1     public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
 2         ListNode res = new ListNode(0);
 3         int num = 0;
 4         int carry = 0;
 5         ListNode current = res;
 6         while((l1 != null) || (l2 != null)) {            
 7             if(l1 != null && l2 != null) {
 8                 num = l1.val + l2.val + carry;
 9                 carry = 0;
10                 if(num > 9) {
11                     carry = 1;
12                     num = num % 10;
13                 }
14                 current.next = new ListNode(num);
15                 l1 = l1.next;
16                 l2 = l2.next;
17                 current = current.next;
18             }else if(l1 == null) {
19                 if(carry == 0) {
20                     num = l2.val;
21                 }else {                    
22                     num = l2.val + carry;
23                     carry = 0;
24                     if(num > 9) {
25                         num = num % 10;
26                         carry = 1;
27                     }
28                 }
29                 current.next = new ListNode(num);
30                 l2 = l2.next;
31                 current = current.next;
32             }else {
33                 if(carry == 0) {
34                     num = l1.val;
35                 }else {                    
36                     num = l1.val + carry;
37                     carry = 0;
38                     if(num > 9) {
39                         num = num % 10;
40                         carry = 1;
41                     }
42                 }
43                 current.next = new ListNode(num);
44                 l1 = l1.next;
45                 current = current.next;
46             }
47         }
48         if(carry == 1) {
　　　　　　　　　//检查最后一次进位
49             current.next = new ListNode(1);
50         }
51         return res.next;
52     }