面试题15：合并两个排序的链表

方法一：非递归

代码：

#include "stdafx.h"
#include <iostream>
using namespace std;

struct ListNode
{
	int m_nValue;
	ListNode *m_pNext;
};

//合并两个有序链表,非递归方法
ListNode *MergeTwoList(ListNode *pListOneHead, ListNode *pListTwoHead)
{
	if (pListOneHead == NULL)
	{
		return pListTwoHead;
	}

	if (pListTwoHead == NULL)
	{
		return pListOneHead;
	}

	ListNode *pNode1 = pListOneHead;
	ListNode *pNode2 = pListTwoHead;
    ListNode *pMergeListHead = NULL;
	ListNode *pCurLastNode = NULL;

	if (pNode1->m_nValue < pNode2->m_nValue)
	{			
		pMergeListHead = pListOneHead;
		pNode1 = pNode1->m_pNext;
		pCurLastNode = pMergeListHead;
	}
	else
	{
		pMergeListHead = pListTwoHead;
		pNode2 = pNode2->m_pNext;
		pCurLastNode = pMergeListHead;
	}

    while (pNode1 != NULL && pNode2 != NULL)
    {
		if (pNode1->m_nValue < pNode2->m_nValue)
		{	
			pCurLastNode->m_pNext = pNode1;
			pCurLastNode = pNode1;
			pNode1 = pNode1->m_pNext;			
		}
		else
		{
			pCurLastNode->m_pNext = pNode2;
			pCurLastNode = pNode2;
			pNode2 = pNode2->m_pNext;			
		}

		if (pNode1 == NULL)
		{
			pCurLastNode->m_pNext = pNode2;				
		}
		
		if (pNode2 == NULL)
		{
			pCurLastNode->m_pNext = pNode1;			
		}
    }

	return pMergeListHead;
}

//创建一个链表，输入从头到尾结点的值，输入-1表示结束
void CreateList(ListNode *& pHead)
{	
	ListNode *pListNode = NULL;
	ListNode *pCurLastNode = NULL;
	bool isHead = true;

	while (1)
	{
		if (isHead)
		{	
			pHead = new ListNode();
			cin >> pHead->m_nValue;
			if (pHead->m_nValue == -1)
			{
				pHead = NULL;
				break;
			}
			pHead->m_pNext = NULL;
			isHead = false;
			pCurLastNode = pHead;
		}
		else
		{
			pListNode = new ListNode();
			cin >> pListNode->m_nValue;
			if (pListNode->m_nValue == -1)
			{
				break;
			}
			pListNode->m_pNext = NULL;
			pCurLastNode->m_pNext = pListNode;
			pCurLastNode = pListNode;	
		}			
	}
}

//从头到尾打印链表
void PrintList(ListNode *&pHead)
{
	if (pHead != NULL)
	{
		ListNode *pCur = pHead;
		while (pCur != NULL)
		{
			cout << pCur->m_nValue << " ";
			pCur = pCur->m_pNext;
		}
		cout << endl;
	}
	else
	{
		cout << "链表为空！" << endl;
	}
}

int _tmain(int argc, _TCHAR* argv[])
{
	ListNode *pList1Head = NULL;
	CreateList(pList1Head);
	PrintList(pList1Head);

	ListNode *pList2Head = NULL;
	CreateList(pList2Head);
	PrintList(pList2Head);

	ListNode *pMergeListHead = MergeTwoList(pList1Head, pList2Head);
	if (pMergeListHead != NULL)
	{
		cout << pMergeListHead->m_nValue << endl;
	}	
	PrintList(pMergeListHead);

	system("pause");
	return 0;
}

方法二：递归

代码：

#include "stdafx.h"
#include <iostream>
using namespace std;

struct ListNode
{
	int m_nValue;
	ListNode *m_pNext;
};

//合并两个有序链表,递归方法
ListNode *MergeTwoList(ListNode *pListOneHead, ListNode *pListTwoHead)
{
	if (pListOneHead == NULL)
	{
		return pListTwoHead;
	}

	if (pListTwoHead == NULL)
	{
		return pListOneHead;
	}

	ListNode *pMergeListHead = NULL;

	if (pListOneHead->m_nValue < pListTwoHead->m_nValue)
	{
        pMergeListHead = pListOneHead;
		pMergeListHead->m_pNext = MergeTwoList(pMergeListHead->m_pNext, pListTwoHead);
	}
	else
	{
		pMergeListHead = pListTwoHead;
		pMergeListHead->m_pNext = MergeTwoList(pListOneHead, pMergeListHead->m_pNext);
	}

	return pMergeListHead;
}

//创建一个链表，输入从头到尾结点的值，输入-1表示结束
void CreateList(ListNode *& pHead)
{	
	ListNode *pListNode = NULL;
	ListNode *pCurLastNode = NULL;
	bool isHead = true;

	while (1)
	{
		if (isHead)
		{	
			pHead = new ListNode();
			cin >> pHead->m_nValue;
			if (pHead->m_nValue == -1)
			{
				pHead = NULL;
				break;
			}
			pHead->m_pNext = NULL;
			isHead = false;
			pCurLastNode = pHead;
		}
		else
		{
			pListNode = new ListNode();
			cin >> pListNode->m_nValue;
			if (pListNode->m_nValue == -1)
			{
				break;
			}
			pListNode->m_pNext = NULL;
			pCurLastNode->m_pNext = pListNode;
			pCurLastNode = pListNode;	
		}			
	}
}

//从头到尾打印链表
void PrintList(ListNode *&pHead)
{
	if (pHead != NULL)
	{
		ListNode *pCur = pHead;
		while (pCur != NULL)
		{
			cout << pCur->m_nValue << " ";
			pCur = pCur->m_pNext;
		}
		cout << endl;
	}
	else
	{
		cout << "链表为空！" << endl;
	}
}

int _tmain(int argc, _TCHAR* argv[])
{
	ListNode *pList1Head = NULL;
	CreateList(pList1Head);
	PrintList(pList1Head);

	ListNode *pList2Head = NULL;
	CreateList(pList2Head);
	PrintList(pList2Head);

	ListNode *pMergeListHead = MergeTwoList(pList1Head, pList2Head);
	if (pMergeListHead != NULL)
	{
		cout << pMergeListHead->m_nValue << endl;
	}	
	PrintList(pMergeListHead);

	system("pause");
	return 0;
}