Leetcode: Merge Two Sorted Lists

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution
{
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
    {
        //helper指向l1和l2指向最小的哪一个，一直循环着找下去
        ListNode *helper = new ListNode(0);
        ListNode *head = helper;
        while (l1 && l2)
        {
            if (l1->val < l2->val)
            {
                helper->next = l1;
                l1 = l1->next;
            }
            else
            {
                helper->next = l2;
                l2 = l2->next;
            }
            helper = helper->next;
        }
        //如果最后l1或者l2还有指向的数据，直接接到helper的后面
        if (l1) helper->next = l1;
        if (l2) helper->next = l2;
        return head->next;
    }
};