21. Merge Two Sorted Lists

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.

原题链接：https://leetcode.com/problems/merge-two-sorted-lists/

思路
就遍历一遍，比较大小依次插入新链表。注意一下某一个链表为空的状态，或者先遍历完。

代码（C）

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) 
{
    struct ListNode *newhead = (struct ListNode *)malloc(sizeof(struct ListNode));
    if (NULL == newhead)
        return NULL;
    struct ListNode *rethead = newhead;

    if(l1 == NULL)
        return l2;
    if(l2 == NULL)
        return l1;

    while(l1 != NULL || l2 != NULL)
    {
        if(l1 == NULL)
        {
            newhead->next = l2;
            l2 = l2->next;
        }
        else if(l2 == NULL)
        {
            newhead->next = l1;
            l1 = l1->next;
        }
        else if(l1->val < l2->val)
        {
            newhead->next = l1;
            l1 = l1->next;
        }
        else 
        {
            newhead->next = l2;
            l2 = l2->next;
        }
        newhead = newhead->next;
    }
    return rethead->next;
}