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

 Solution 1:

/**
 * 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 cnt=0,size_l1=0,size_l2=0;
        ListNode* res=new ListNode(0);
        ListNode* t=l1;
        int carry=0;
        while(t){
            size_l1++;
            t=t->next;
        }
        t=l2;
        while(t){
            size_l2++;
            t=t->next;
        }

        ListNode* s=res;
        ListNode* c;
        while(cnt<min(size_l1,size_l2)){
            int sum=l1->val+l2->val+carry;
            s->val=sum%10;
            if(s->next==NULL)c=s;
            s=s->next;            
            l1=l1->next;
            l2=l2->next;
            carry=sum/10;
            cnt++;
        }
        if (size_l1>size_l2){
            while(s){
                int sum=s->val+carry;
                s->val=sum%10;
                if(s->next==NULL)c=s;
                s=s->next;
                carry=sum/10;
            }            
        }
        if (size_l1<size_l2){
            while(s){
                int sum=s->val+carry;
                s->val=sum%10;
                if(s->next==NULL)c=s;
                s=s->next;
                carry=sum/10;
            }            
        }
        if (carry) c->next=new ListNode(carry);
        return res;
    }
};

Solution2: use a dummyhead to avoid writing extra conditional statements thus simplify the code. https://leetcode.com/problems/add-two-numbers/solution/

/**
 * 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) {
        ListNode* dummyhead=new ListNode(0);
        ListNode* p1=l1,*p2=l2,*curr=dummyhead;
        int carry=0;
        while(p1||p2){
            int x=(p1==NULL)?0:p1->val;
            int y=(p2==NULL)?0:p2->val;
            int sum=x+y+carry;
            curr->next=new ListNode(sum%10);
            carry=sum/10;
            curr=curr->next;
            if(p1!=NULL) p1=p1->next;
            if(p2!=NULL) p2=p2->next;
        }
        if (carry) curr->next=new ListNode(carry);
        return dummyhead->next;
    }
};