Add Two Numbers

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
12         if( !l1 ) return l2;    //l1为空
13         if( !l2 ) return l1;    //l2为空
14         ListNode node(0);   //建立一个头结点
15         ListNode* pre = &node;  //前驱节点
16         int carry = 0;  //进位值
17         while( l1 && l2 )  {    //当两链表所指的节点都存在时
18             int sum = l1->val + l2->val + carry;    //模拟加法
19             ListNode* p = new ListNode( sum%10 );
20             carry = sum/10;
21             pre->next = p;
22             pre = p;
23             l1 = l1->next;
24             l2 = l2->next;
25         }
26         l1 = l1 ? l1 : l2;  //使l1指向不为空的链表
27         while( l1 ) {   //处理进位有可能导致高位数变化
28             int sum = l1->val + carry;
29             ListNode* p = new ListNode( sum%10 );
30             carry = sum/10;
31             pre->next = p;
32             pre = p;
33             l1 = l1->next;
34         }
35         if( carry ) {   //如果依然还有进位
36             ListNode* p = new ListNode(carry);
37             pre->next = p;
38         }
39         return node.next;
40     }
41 };