剑指Offer面试题5 从头到尾打印链表

题目：输入一个链表的头结点，从尾到头反过来打印出每个结点的值。

链表结点定义如下：

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

思路一：将链表中的连接指针反转过来，改变链表的方向就可以从头到尾输出了。不足：这样做会改变原来链表的结构。

思路二：遍历链表，遍历顺序是从头到尾而输出顺序却是从尾到头。也就是说第一个遍历到的结点最后一个输出，可以用栈实现。每当遍历到一个结点的时候，将该结点入栈。链表遍历结束后再从栈顶开始逐个输出结点的值。

思路三：用递归来实现。每访问到一个结点的时候，先递归输出它后面的结点在输出自己本身，这样链表的输出结果就反过来了。

思路二代码：

#include <iostream>
#include <stdlib.h>
#include <stack>
using namespace std;

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

ListNode *CreateList()
{
	ListNode *pHead = NULL;
	ListNode *pTail = NULL;
	ListNode *pTemp = NULL;

	int nNum;
	cin >> nNum;

	while(nNum != 0)
	{
		pTemp = (ListNode*)malloc(sizeof(ListNode));
		pTemp->m_nKey = nNum;
		pTemp->m_pNext = NULL;
		if(pHead == NULL)
		{
			pHead = pTemp;
		}
		else
		{
			pTail->m_pNext = pTemp;
		}
		pTail = pTemp;

		cin >> nNum;
	}
	return pHead;
}

void PrintListReversingly_Iteratively(ListNode* pHead)
{
    stack<ListNode*> nodes;
	ListNode* pNode = pHead;
    while(pNode != NULL)
    {
        nodes.push(pNode);
        pNode = pNode->m_pNext;
    }
	while(!nodes.empty())
    {
        pNode = nodes.top();
        cout << pNode->m_nKey <<endl;
        nodes.pop();
    }
}

void Print(ListNode *pHead)
{
	if(pHead == NULL) return ;
	while(pHead)
	{
		cout << pHead->m_nKey << endl;
		pHead = pHead->m_pNext;
	}
}

int main()
{
    ListNode* pHead = NULL;
	pHead = CreateList();
	Print(pHead);
	PrintListReversingly_Iteratively(pHead);
		
    return 0;
}

思路三代码：

#include <iostream>
#include <stdlib.h>
using namespace std;

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

ListNode *CreateList()
{
	ListNode *pHead = NULL;
	ListNode *pTail = NULL;
	ListNode *pTemp = NULL;

	int nNum;
	cin >> nNum;

	while(nNum != 0)
	{
		pTemp = (ListNode*)malloc(sizeof(ListNode));
		pTemp->m_nKey = nNum;
		pTemp->m_pNext = NULL;
		if(pHead == NULL)
		{
			pHead = pTemp;
		}
		else
		{
			pTail->m_pNext = pTemp;
		}
		pTail = pTemp;

		cin >> nNum;
	}
	return pHead;
}

void PrintListReversingly_Iteratively(ListNode* pHead)
{
    if(pHead != NULL)
    {
        if(pHead->m_pNext != NULL)
        {
            PrintListReversingly_Iteratively(pHead->m_pNext);
        }
        cout << pHead->m_nKey << endl;
    }
}

void Print(ListNode *pHead)
{
	if(pHead == NULL) return ;
	while(pHead)
	{
		cout << pHead->m_nKey << endl;
		pHead = pHead->m_pNext;
	}
}

int main()
{
    ListNode* pHead = NULL;
	pHead = CreateList();
	Print(pHead);
	PrintListReversingly_Iteratively(pHead);
	
    return 0;
}