Leetcode 2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. 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.

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

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

题意不讲了，直接上答案。
这道题给的启示就是注意头结点head的设置，设置了head结点，就可以避免当前节点为null的情况，否者无法避免。

/*class ListNode {      int val;      ListNode next;      ListNode(int x) { val = x; }  }*//* * 这道题还是简单的，只不过是遍历链表，双指针遍历即可， *  * 注意要设置head结点 *  * */public class Solution {     public ListNode addTwoNumbers(ListNode l1, ListNode l2)      {         ListNode res=new ListNode(-1);         if(l1==null)             return l2;         else if(l2==null)             return l1;         ListNode iter=res,iter1=l1,iter2=l2;         int carry=0;         while(iter1!=null || iter2!=null)         {             int x = iter1!=null ? iter1.val : 0;             int y = iter2!=null ? iter2.val : 0;             int sum=x+y+carry;             carry=sum/10;             iter.next=new ListNode(sum%10);             iter=iter.next;             if(iter1!=null)                 iter1=iter1.next;             if(iter2!=null)                 iter2=iter2.next;         }         if(carry>0)             iter.next=new ListNode(carry);         return res.next;     }}

下面是C++版本答案

/*struct ListNode{      int val;      ListNode *next;      ListNode(int x) : val(x), next(NULL) {}};*/class Solution{public:    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2)    {        int jinwei=0;        ListNode* head=new ListNode(-1);        ListNode* iter1=l1;        ListNode* iter2=l2;        ListNode* iter3=head;        while(iter1!=NULL && iter2!=NULL)        {            int sum = iter1->val + iter2->val + jinwei;            jinwei=sum/10;            sum=sum%10;            iter3->next = new ListNode(sum);            iter3=iter3->next;            iter1=iter1->next;            iter2=iter2->next;        }        while(iter1!=NULL)        {            int sum = iter1->val + jinwei;            jinwei=sum/10;            sum=sum%10;            iter3->next = new ListNode(sum);            iter3=iter3->next;            iter1=iter1->next;        }        while(iter2!=NULL)        {            int sum = iter2->val + jinwei;            jinwei=sum/10;            sum=sum%10;            iter3->next = new ListNode(sum);            iter3=iter3->next;            iter2=iter2->next;        }        if(jinwei>0)             iter3->next = new ListNode(jinwei);        return head->next;    }};