leetcode2.两数相加

/**
 * 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 = (ListNode *)malloc(sizeof(ListNode));
        ListNode *p = head;
        int cf = 0;
        while(l1 || l2){
            // 为空指针时，认为取到为0
            int num1 = l1 ? l1->val : 0;
            int num2 = l2 ? l2->val : 0;
            int directNum = num1 + num2 + cf;
            // error1:cf 在mod10后才加上
            int curVal =  (directNum) % 10 ;
            // printf("curVal:%d,cf:%d\n" , curVal, cf);
            cf = directNum >= 10 ? 1 : 0;
            ListNode *curNode = new ListNode(curVal);
            p->next = curNode;
            p = p->next;
            // error2: l1 l2 是空指针时，不可继续用next了
            if(l1) l1 = l1->next;
            if(l2) l2 = l2->next;
        }
        // 最后两个数字相加后产生进位
        if(cf == 1){
            ListNode *curNode = new ListNode(cf);
            p->next = curNode;
            p = p->next;
        }
        p->next = NULL;
        return head->next;
    }
};