LeetCode-2. 两数相加

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
       ListNode* head = new ListNode(-1); //头指针
       ListNode* h = head; //移动指针
       int sum = 0;
       int carry = 0; //进位
       while(l1 != nullptr || l2 != nullptr)
       {
           sum = 0;   //每一位上sum都得重新赋值为0
           if(l1)
           {
               sum += l1->val;
               l1 = l1->next;
           }
           if(l2)
           {
               sum += l2->val;
               l2 = l2->next;
           }
           if(carry)
           {
               sum++;
           }
           h->next = new ListNode(sum % 10);
           h = h->next;
           carry = sum >= 10 ? 1:0; 
       }
       if(carry)   //考虑最高位单独是1的情况
       {
           h->next = new ListNode(1);
       }
       return head->next;
    }
};