2_Add Two Numbers

/**
 * 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 *result = new ListNode(0);  //return listNode
        ListNode *p = l1, *q = l2, *r = result;  //linkedList pointer
        int carry = 0; //計算有沒有進位，有進位要新增一個node
        int sum = 0; 
        
        while(p != NULL && q != NULL){   
            
            // l1==l2
            sum = p->val + q->val + carry; 
            carry = sum / 10;
            r->next = new ListNode(sum % 10); 
            p = p->next;  //移動指標
            q = q->next;
            r = r->next; 
        }
        
        while(p != NULL || q != NULL || carry){ 
            
            //  l1 !=l2 或者是l1=l2但會進位，例如[5][5]      
            sum = carry;
            
            if(p != NULL){
                
                // l1 較多
                sum = sum + p->val;
                p = p->next;
            }
            if(q != NULL){
                
                //  l2 較多
                sum = sum + q->val;
                q = q->next;
            }
       
            carry = sum / 10;
            // 會進到while loop，一定需要新增node，所以可以最後做
            r->next = new ListNode(sum % 10);
            // 記得移動指標
            r = r -> next;
        } 
        return result->next;  
    }
};