算法练习2之单链表求和

笔试题目：

1、用单向链表表示十进制整数，求两个正整数的和。如下图，1234+34=1268，
注意:单向链表的方向，不允许使用其他的数据结构。

题目分析：

题目中提到了，数据结构只能使用单链表，所以数组不在考虑范围之内。
因为将数字转为单链表以后，最高位排在表头，而我们进行整数加法的时候，是从个位开始的，与单链表的顺序相反。所以我们考虑对链表进行反转，然后再做加法。

其中反转和求和的示意图如下：

对求和以后的结果再进行反转：

下面是C++的实现

代码解读

1.节点的数据结构定义如下:

//节点定义
struct Node_t
{
    int m_value;
    Node_t * m_pNext;
};

2. 将int转为List

//int转List
Node_t* IntToNodeList(int value)
{
    Node_t* pHead= new Node_t;
    pHead->m_pNext=nullptr;
    while(value > 0)
    {
        Node_t * pNewNode = new Node_t;
        pNewNode->m_value = value % 10;
        pNewNode->m_pNext = pHead->m_pNext;
        pHead->m_pNext = pNewNode; 
        value = value / 10;
    }

    Node_t * pResult = pHead->m_pNext;
    pHead->m_pNext=nullptr;
    delete pHead;
    return pResult;
}

3.将List转为int

//List转int
int NodeListToInt(Node_t* pList)
{
    Node_t * pHead = pList;
    int nResult = 0;
    while(( pHead != nullptr)){
        nResult = nResult*10+pHead->m_value;
        pHead = pHead->m_pNext;
    }
    return nResult;
}

4. 释放节点内存

//释放节点
void FreeNodeList(Node_t* pList)
{
    Node_t* pHead = pList;
    Node_t* pFree = nullptr;
    while(pHead)
    {
        pFree = pHead;
        pHead = pHead->m_pNext;
        delete pFree;
    }
}

5 .实现链表的反转

//链表翻转,使用传入的链表的内存
Node_t* ReverseList(Node_t* pList)
{
    Node_t * pResult = nullptr;
    Node_t * pTem = nullptr;
    while(pList != nullptr)
    {
        pTem = pList->m_pNext;
        pList->m_pNext=pResult;
        pResult= pList;
        pList = pTem;   
    }
    return pResult;
}

6.将链表相加

//链表相加
Node_t * AddList(Node_t* pLeft,Node_t* pRight)
{
    Node_t * pRevLeft = ReverseList(pLeft);
    Node_t * pRevRight = ReverseList(pRight);
    Node_t * pResult=nullptr;
    int toUpValue = 0;//进位
    int nLeftValue = 0;
    int nRightValue = 0;

    while(pRevLeft != nullptr || pRevRight != nullptr)
    {
        Node_t * pNewNode = new Node_t;
        pNewNode->m_pNext = pResult;
        pResult = pNewNode;

        nRightValue = 0;
        nLeftValue = 0;

        if(pRevLeft)
        {
            nLeftValue = pRevLeft->m_value;
            pRevLeft = pRevLeft->m_pNext;
        }
        
        if(pRevRight)
        {
            nRightValue = pRevRight->m_value;
            pRevRight = pRevRight->m_pNext;
        }
        

        int curPosSum = nRightValue+nLeftValue;
        pNewNode->m_value = curPosSum%10+toUpValue;
        toUpValue = curPosSum/10;
    }
    return pResult;
}

主函数：

int main(int argc,char * argv[])
{
    int nLeftValue = rand();
    int nRightValue = rand();
    std::cout<<"Test Reverst"<<std::endl;
    Node_t * pLeftList = IntToNodeList(nLeftValue);
    Node_t * pRightList = IntToNodeList(nRightValue);
    Node_t * pSumList = AddList(pLeftList,pRightList);
    int nSum = NodeListToInt(pSumList);

    FreeNodeList(pLeftList);
    FreeNodeList(pRightList);
    FreeNodeList(pSumList);
    std::cout<<"TEST "<<nRightValue<<"   "<<nLeftValue <<"           "<<nSum<<std::endl;
    return 0;
}