经典算法——合并两个有序链表

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.

完整测试程序：

#include <iostream>
using namespace std;

struct ListNode
{
	int   val;
	ListNode* next;
};

ListNode* MergeTwoSortedLists(ListNode* l1, ListNode* l2)
{
	ListNode* newhead = NULL;
	ListNode* p = NULL;
	if (l1 == NULL) return l2;
	if (l2 == NULL) return l1;
	if (l1->val < l2->val){ newhead = l1;  p = l1; l1 = l1->next; }
	else{ newhead = l2; p = l2; l2 = l2->next; }

	while (l1 && l2)
	{
		if (l1->val < l2->val)
		{
			p ->next= l1;
			l1 = l1->next;
		}
		else
		{
			p->next = l2;
			l2 = l2->next;
		}
		p = p->next;
	}
	p->next = l1 ? l1 : l2;
	return newhead;
}

ListNode* CreateListNode(int value)
{
	ListNode* pNode = new ListNode();
	pNode->val = value;
	pNode->next = NULL;

	return pNode;
}

void DestroyList(ListNode* pHead)
{
	ListNode* pNode = pHead;
	while (pNode != NULL)
	{
		pHead = pHead->next;
		delete pNode;
		pNode = pHead;
	}
}

void ConnectListNodes(ListNode* pCurrent, ListNode* pNext)
{
	if (pCurrent == NULL)
	{
		printf("Error to connect two nodes.\n");
		exit(1);
	}
	pCurrent->next = pNext;
}

int  main()
{
	ListNode* pNode1 = CreateListNode(1);
	ListNode* pNode2 = CreateListNode(2);
	ListNode* pNode3 = CreateListNode(3);
	ListNode* pNode4 = CreateListNode(4);
	ListNode* pNode5 = CreateListNode(5);

	ListNode* pNode6 = CreateListNode(3);
	ListNode* pNode7 = CreateListNode(5);

	ConnectListNodes(pNode1, pNode2);
	ConnectListNodes(pNode2, pNode3);
	ConnectListNodes(pNode3, pNode4);
	ConnectListNodes(pNode4, pNode5);

	ConnectListNodes(pNode6, pNode7);
	ListNode* p1 = pNode1;
	ListNode* p6 = pNode6;
	cout << "链表l1： ";
	while (p1)
	{
		cout << p1->val << " ";
		p1 = p1->next;
	}
	cout << endl;
	cout << "链表l2： ";
	while (p6)
	{
		cout << p6->val << " ";
		p6 = p6->next;
	}
	cout << endl;

	ListNode* res = MergeTwoSortedLists(pNode1, pNode6);

	cout << "合并后链表： ";
	while (res)
	{
		cout << res->val << " ";
		res = res->next;
	}
	cout << endl;
	system("pause");
	DestroyList(res);
	return 0;
}