[算法练习] 反转链表

题目说明：

输入一个链表的头结点，反转该链表，并返回反转后链表的头结点。链表结点定义如下：
struct ListNode
{
     int m_nKey;
     ListNode* m_pNext;
};

程序代码：

#include <gtest/gtest.h>
#include <vector>
using namespace std;

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

ListNode* ReverseLinkedList(ListNode* pHead)
{
    ListNode* pNext = pHead;
    ListNode* pPrev = NULL;

    while (pNext)
    {
        ListNode* pTemp = pNext->m_pNext;
        pNext->m_pNext = pPrev;
        pPrev = pNext;
        pNext = pTemp;
    }

    return pPrev;
}

// 递归方式实现
ListNode* ReverseLinkedList2(ListNode* pHead, ListNode* pPrev = NULL)
{
    ListNode* pReversedHead = pHead;
    if (pHead)
    {
        if (pHead->m_pNext)
        {
            pReversedHead = ReverseLinkedList2(pHead->m_pNext, pHead);
        }

        pHead->m_pNext = pPrev;
    }

    return pReversedHead;
}

ListNode* CreateLinkedList(const string& data)
{
    ListNode* pHead = NULL;
    ListNode* pLast = NULL;
    for (int i=0; i<data.size(); ++i)
    {
        ListNode* pNext = new ListNode();
        pNext->m_nKey = data[i];
        pNext->m_pNext = NULL;

        if (!pHead)
        {
            pHead = pLast = pNext;
        }
        else
        {
            pLast->m_pNext = pNext;
            pLast = pNext;
        }
    }

    return pHead;
}

void ReleaseLinkedList(ListNode* pHead)
{
    ListNode* pTemp = NULL;
    while (pHead)
    {
        pTemp = pHead;
        pHead = pHead->m_pNext;
        delete pTemp;
    }
}

string ConvertLinkedList(ListNode* pHead)
{
    string strValue;
    while (pHead)
    {
        strValue+= (char)pHead->m_nKey;
        pHead = pHead->m_pNext;
    }

    return strValue;
}

TEST(Pratices, tReverseLinkedList)
{
    // NULL
    ListNode* pHead = NULL;
    pHead = ReverseLinkedList(pHead);
    ASSERT_TRUE(!pHead);

    // a -> a
    pHead = CreateLinkedList("a");
    pHead = ReverseLinkedList(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"a");
    ReleaseLinkedList(pHead);

    // ab -> ba
    pHead = CreateLinkedList("ab");
    pHead = ReverseLinkedList(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"ba");
    ReleaseLinkedList(pHead);

    // abcedfg -> gfdecba
    pHead = CreateLinkedList("abcedfg");
    pHead = ReverseLinkedList(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"gfdecba");
    ReleaseLinkedList(pHead);

    /// 递归逆序测试
    pHead = NULL;
    pHead = ReverseLinkedList2(pHead);
    ASSERT_TRUE(!pHead);

    // a -> a
    pHead = CreateLinkedList("a");
    pHead = ReverseLinkedList2(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"a");
    ReleaseLinkedList(pHead);

    // ab -> ba
    pHead = CreateLinkedList("ab");
    pHead = ReverseLinkedList2(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"ba");
    ReleaseLinkedList(pHead);

    // abcedfg -> gfdecba
    pHead = CreateLinkedList("abcedfg");
    pHead = ReverseLinkedList2(pHead);
    ASSERT_EQ(ConvertLinkedList(pHead),"gfdecba");
    ReleaseLinkedList(pHead);
}