Merge Two Sorted Lists

从前往后Merge，时间O(1)，空间O(n)。代码如下：

class Solution {public:    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {        ListNode dummy(-1);        ListNode *cur = &dummy;                while(l1 && l2)        {            if(l1->val > l2->val) cur->next = l2, cur = l2, l2 = l2->next;            else                  cur->next = l1, cur = l1, l1 = l1->next;        }                if(l1) cur->next = l1;        if(l2) cur->next = l2;                return dummy.next;    }};