Leetcode (2) Merge Two Sorted Lists

题目要求：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.

简单的说就是将量和排好序的链表合并为一个链表。这道题的考察点为链表的操作。需要注意的是对链表有效性的判断，代码主要分为三部分，第一部分对输入指针的判断，第二部分是根据链表节点大小进行合并，第三部分是当一个链表达到最后节点之后，将没达到最后节点的链表链接到返回链表的后面，这样可以减少后面部分的循环操作。需要注意的是对于返回链表需要保存其头指针。

/**
 * 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;

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

	 if (l1 != NULL)
	     l->next = l1;
	 else if (l2 != NULL)
	     l->next = l2;

	 return head;
    }
};

Leetcode (2) Merge Two Sorted Lists

原文：http://blog.csdn.net/angelazy/article/details/44995369