21. Merge Two Sorted Lists

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本文介绍了一种合并两个已排序链表的方法，并提供了一个C++实现示例。该方法通过比较两个链表节点的值来合并链表，创建一个新的已排序链表。

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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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4
class Solution {
public:
	ListNode * mergeTwoLists(ListNode* l1, ListNode * l2)
	{
		if (l1 == NULL) return l2;
		if (l2 == NULL)  return l1;
		ListNode * p1 = l1, *p2 = l2, *p, *head = NULL, *q = NULL;
		while (p1 != NULL && p2 != NULL)
		{
			if (p1->val <= p2->val)
			{
				p = new ListNode(p1->val);
				p1 = p1->next;
			}
			else
			{
				p = new ListNode(p2->val);
				p2 = p2->next;
			}

			if (head == NULL)
			{
				head = p;
			}
			else
			{
				q->next = p;
			}
			q = p;
		} 
		if (p1 != NULL) q->next = p1;
		else
			q->next = p2; 
		return head;
	}

};