Merge Two Sorted Lists

题目：
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.

题意：
合并两个排序过的链表

思路：
这是个非经常见的问题，须要注意的就是给定两个链表是否为空。

代码：

/**
 * 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) {
        if(l1 == NULL && l2 == NULL){
            return NULL;
        }else if(l1 == NULL && l2 != NULL){
            return l2;
        }else if(l1 != NULL && l2 == NULL){
            return l1;
        }
        //指定头节点
        ListNode *new_head = NULL;
        if(l1->val < l2->val){
            new_head = l1;
            l1 = l1->next;
        }else{
            new_head = l2;
            l2 = l2->next;
        }
        ListNode *p = new_head;
        while(l1 != NULL && l2 != NULL){
            if(l1->val < l2->val){
                p->next = l1;
                p = p->next;
                l1 = l1->next;
            }else{
                p->next = l2;
                p = p->next;
                l2 = l2->next;
            }
        }
        //接上余下的链表段
        if(l1 != NULL){
            p->next = l1;
        }else{
            p->next = l2;
        }
        return new_head;
    }
};