[LeetCode] 21. Merge Two Sorted Lists

本题算法很简单，但利用引用保存头结点的方法值得学习，代码如下：

/**
 * 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 head(INT_MIN);
        ListNode* res = &head;
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                res->next = l1;
                l1 = l1->next;
            }
            else
            {
                res->next = l2;
                l2 = l2->next;
            }
            res = res->next;
        }
        res->next = l1 ? l1 : l2;
        return head.next;
    }
};