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

大数相加，只保留各位，进位加到下一节点。

需注意，判断l1,l2是否为空。若两链表长度不同，需将剩余节点同样作进位处理，若最后剩有进位，需增加新节点。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if(l1==NULL) return l2;
        if(l2==NULL) return l1;
        ListNode* res=NULL,*pNode=NULL,*pnext=NULL;
        ListNode* p=l1,*q=l2;
        int up=0;
        while(p!=NULL&&q!=NULL)
        {
            pnext=new ListNode(p->val+q->val+up);
            up=pnext->val/10;
            pnext->val=pnext->val%10;
            if(res==NULL)
              res=pNode=pnext;
            else
              {
                  pNode->next=pnext;
                  pNode=pnext;
              }
            p=p->next;
            q=q->next;
        }
        while(p!=NULL)
        {
            pnext=new ListNode(p->val+up);
            up=pnext->val/10;
            pnext->val=pnext->val%10;
            pNode->next=pnext;
            pNode=pnext;
            p=p->next;
        }
        while(q!=NULL)
        {
            pnext=new ListNode(q->val+up);
            up=pnext->val/10;
            pnext->val=pnext->val%10;
            pNode->next=pnext;
            pNode=pnext;
            q=q->next;
        }
        if(up!=0)
        {
            pnext=new ListNode(up);
             pNode->next=pnext;
            pNode=pnext;
        }
        return res;
    }
};

 另一种写法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        int add = 0;
        ListNode *res = NULL;
        if(l1==NULL&&l2==NULL)
            return res;
        ListNode *p = new ListNode(0);
        res = p;
        while(l1!=NULL&&l2!=NULL)
        {
            int val = l1->val+l2->val+add;
            add = 0;
            p->next=new ListNode(val%10);
            add=val/10;
            p=p->next;
            l1=l1->next;
            l2=l2->next;
        }
        while(l1!=NULL)
        {
            int val = l1->val+add;
            add=0;
            p->next=new ListNode(val%10);
            add=val/10;
            p=p->next;
            l1=l1->next;
        }
        while(l2!=NULL)
        {
            int val = l2->val+add;
            add=0;
            p->next=new ListNode(val%10);
            add=val/10;
            p=p->next;
            l2=l2->next;
        }
        if(add!=0)
        {
            p->next=new ListNode(add);
        }
        return res->next;
    }
};