刷题（牛客网）NC2 重排链表

题目：原单链表： {L(0), L(1), L(2)..., L(n-2), L(n-1), L(n)}， 重新排序为：{L(0), L(n), L(1), L(n-1), L(2), L(n-2), ..., L(m)}

输入：n( n: [0, 20000])个数字：{L(0), L(1), L(2)..., L(n-2), L(n-1), L(n)}( L(n): [0, 1000] ), 输入顺序即为链表元素顺序。

输出：重排后的链表：{L(0), L(n), L(1), L(n-1), L(2), L(n-2), ..., L(m)}

限制：空间复杂度 O(1)， 时间复杂度 O(n)

解题语言：C语言

题目可看成是 对后半部分的结点逆序，逐个插入前半部分的结点中间，则问题拆解成两个问题：1）找链表的中结点位置；2）对后半部分结点逆序重排。

问题1）可利用快慢指针实现；问题2）可利用栈的特性：先入后出；

解题后的心得：

逆序问题可首先考虑栈，空间复杂度和时间复杂度均为最佳。

代码：

1. 变量定义 和 main函数

#include 
#include 

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

int main(int argc, char *argv[])
{
    struct ListNode *pList = NULL;

    pList = createList();

    reorderList(pList);

    printList(pList);

    return 0;
}

2. 链表创建和打印输出：

struct ListNode *createList()
{
    struct ListNode *pNew  = NULL;
    struct ListNode *pHead = NULL;
    struct ListNode *pEnd  = NULL;

    unsigned int inputNumCount = 0;
    unsigned int inputNumber   = 0;
    unsigned int count         = 0;

    printf("please input number count: ");
    scanf("%u", &inputNumCount);

    if (0 == inputNumCount)
    {
        return NULL;
    }
    
    pHead = NULL;
    pEnd  = NULL;
    count = 0;
    while (inputNumCount--)
    {
        printf("%u: ", count+1);
        scanf("%u", &inputNumber);

        pNew = (struct ListNode *)malloc(sizeof(struct ListNode));
        if (NULL == pNew)
        {
            printf("malloc node error! inputNumCount: %un", inputNumCount);
            return pNew;
        }
        pNew->val  = inputNumber;
        pNew->next = NULL;
        count++;

        if (1 == count)
        {
            pHead = pNew;
            pEnd  = pNew;
        }
        else
        {
            pEnd->next = pNew;
            pEnd = pNew;
        }
    }

    return pHead;
}

void printList(struct ListNode *head)
{
    struct ListNode *plistNode = NULL;

    if (NULL == head)
    {
        printf("list is NULLn");
        return ;
    }

    plistNode = head;
    while (NULL != plistNode)
    {
        printf("%d ", plistNode->val);
        plistNode = plistNode->next;
    }
    printf("n");
    
    return ;
}

 3. 重排列表接口：（编码习惯：栈空间最后要销毁，避免内存泄漏）

void reorderList(struct ListNode *head) 
{
    struct ListNode *pFirstHalfList  = NULL;
    struct ListNode *pSecondHalfList = NULL;
    struct ListNode *pTmpListNode    = NULL;

    struct ListNode *pFastNode    = NULL;
    struct ListNode *pSlowNode    = NULL;

    struct ListNode *pStackTop     = NULL;
    struct ListNode *pStackBottom  = NULL;
    struct ListNode *pTmpStackNode = NULL;

    if (NULL == head)
    {
        //printf("list is not exit!n");
        return ;
    }

    if (NULL == head->next ||
        NULL == head->next->next)
    {
        //printf("list not need sortn");
        return ;
    }

    
    pSlowNode = head;
    pFastNode = head->next;
    while(NULL != pFastNode->next &&
          NULL != pFastNode->next->next)
    {
        pSlowNode = pSlowNode->next;
        pFastNode = pFastNode->next->next;
    }

    
    pFirstHalfList  = head;
    pSecondHalfList = pSlowNode->next;
    pSlowNode->next = NULL;
    
    
    pStackBottom = (struct ListNode *)malloc(sizeof(struct ListNode));
    if (NULL == pStackBottom)
    {
        printf("creat stack failed!n");
        return ;
    }
    memset(pStackBottom, 0, sizeof(struct ListNode));
    pStackTop = pStackBottom;

    
    while(NULL != pSecondHalfList)
    {
        pTmpListNode = pSecondHalfList->next;

        pSecondHalfList->next = pStackTop;
        pStackTop = pSecondHalfList;

        pSecondHalfList = pTmpListNode;
    }

    
    while (pStackTop != pStackBottom &&
           NULL != pFirstHalfList)
    {
        pTmpListNode   = pFirstHalfList;
        pTmpStackNode  = pStackTop;

        pFirstHalfList = pFirstHalfList->next;
        pStackTop      = pStackTop->next;

        pTmpStackNode->next = pTmpListNode->next;
        pTmpListNode->next  = pTmpStackNode;
    }

    if (pStackTop != pStackBottom)
    {
        pTmpStackNode->next = pStackTop;
        pStackTop = pStackTop->next;

        pTmpStackNode->next->next = NULL;
    }
    
    
    while (pStackTop != pStackBottom)
    {
        pTmpStackNode = pStackTop;
        pStackTop = pStackTop->next;

        printf("code error!!n");
        free(pTmpStackNode);
    }

    free(pStackBottom);

    return ;    
}