LeetCode 2. 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

A small trick will make life easier :)

 ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if(!l1) return l2;
        if(!l2) return l1;
        ListNode* l1_node = l1;
        ListNode* l2_node = l2;
        
        ListNode* dummy = new ListNode(0);
        ListNode* newHead = dummy;
        int overflow = 0;
        while(l1_node || l2_node) {
            int sum = (l1_node != NULL ? l1_node->val : 0) + (l2_node != NULL ? l2_node->val : 0) + overflow;  // <img alt="微笑" src="http://static.blog.youkuaiyun.com/xheditor/xheditor_emot/default/smile.gif" />
            overflow = sum / 10;
            ListNode* tmp = new ListNode(sum % 10);
            dummy->next = tmp;
            dummy = tmp;
            if(l1_node)
                l1_node = l1_node->next;
            if(l2_node)
                l2_node = l2_node->next;
        }
        if(overflow) dummy->next = new ListNode(overflow);
        ListNode* head = newHead->next;
        delete newHead;  // in case of overflow.... need to take good care of memory bit.
        return head;
    }