LeetCode-02两数相加

/**
 * 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 preHead(0), *p = &preHead;
    int extra = 0;
    while (l1 || l2 || extra) {
        int sum = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + extra;
        extra = sum / 10;
        p->next = new ListNode(sum % 10);
        p = p->next;
        l1 = l1 ? l1->next : l1;
        l2 = l2 ? l2->next : l2;
    }
    return preHead.next;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode *preHead = (struct ListNode *)malloc(sizeof(struct ListNode));
    preHead->val = 0;   
    struct ListNode *p = NULL;
    int extra = 0;
    while (l1 || l2 || extra) {
        if(p == NULL)
        {
            p=preHead;
        }
        else
        {
            p->next=(struct ListNode*)malloc(sizeof(struct ListNode)); //这里写成p会出错
            p->next->val=0;
            p=p->next;
        }
        int sum = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + extra;
        extra = sum / 10;
        p->val = sum % 10;
        p->next=NULL;//不能开始就p=p->next;没分配内存会报错
        l1 = l1 ? l1->next : l1;
        l2 = l2 ? l2->next : l2;
    }
    return preHead;
    
}