Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

题目解析：

合并链表，很简单，注意好传入为空指针即可。

由于头结点的确定，为了避免在循环中做无谓的比较，直接在while之前先确定头指针。

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        if(l1 == NULL)
            return l2;
        if(l2 == NULL)
            return l1;
        ListNode *head;
        if(l1->val < l2->val){
            head = l1;
            l1 = l1->next;
        }else{
            head = l2;
            l2 = l2->next;
        }

        ListNode *p = head;
        while(l1 && l2){
            if(l1->val < l2->val){
                p->next = l1;
                p = l1;
                l1 = l1->next;
            }else{
                p->next = l2;
                p = l2;
                l2 = l2->next;
            }
        }
        if(l1 == NULL){
            p->next = l2;
        }
        if(l2 == NULL)
            p->next = l1;
        return head;
    }
};