[LeetCode] 005: Add Two Numbers

[Problem]

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

[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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        // ignore null lists
        if(l1 == NULL && l2 == NULL) return NULL;
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        
        // add
        int carry = 0;
        ListNode* p = NULL;
        ListNode* head = NULL;
        while(l1 != NULL || l2 != NULL){
            // add
            int sum = carry;
            if(l1 != NULL){
                sum += l1->val;
                l1 = l1->next;
            }
            if(l2 != NULL){
                sum += l2->val;
                l2 = l2->next;
            }
            
            // update carry and sum
            carry = sum / 10;
            sum = sum % 10;
            if(p == NULL){
                p = new ListNode(sum);
                head = p;
            }
            else{
                ListNode *node = new ListNode(sum);
                p->next = node;
                p = p->next;
            }
        }
        
        // be careful the last carry
        if(carry > 0){
            ListNode *node = new ListNode(carry);
            p->next = node;
        }
        return head;
    }
};

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