leetcode_merge_two_sorted_lists

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难度：Easy

注意：new一个ListNode，返回的是ListNode的指针；直接调用ListNode构造函数，返回的是ListNode。
代码如下：

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/**
 * 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) {
        ListNode* dummy = new ListNode(-1);
        ListNode* cur = dummy;
        while(l1 != NULL && l2 != NULL)
        {
            if(l1->val < l2->val)
            {
                cur->next = l1;
                cur = cur->next;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                cur = cur->next;
                l2 = l2->next;
            }
        }
        if(l1 != NULL)
        {
            cur->next = l1;
        }
        if(l2 != NULL)
        {
            cur->next = l2;
        }
        return dummy->next;
    }
};

运行结果：9ms，超过23.48%