排序算法---链表反转和成环判断

1.链表反转

　　思想：正向遍历链表，一次取出每一个节点，然后以头插的方式将节点插入，就可以逆序

2.成环判断

　　思想：定义两个指针，然后遍历链表，一个指针一次走一个节点，另一个指针一次走两个节点，如果链表成环，那么两个指针肯定有取值相等的情况；否则，遍历正常完成。

#include <iostream>

using namespace std;

struct Node
{
    struct Node *next;
    int data;
};

Node *createNode(int x)
{
    Node *p = NULL;
    
    p = new Node;
    p->next = NULL;
    p->data = x;
    
    return p;
}

void addNode(Node **head, Node *p)
{
    Node *temp = NULL;
    if(*head == NULL)
    {
        *head = p;
    }
    else
    {
        temp = *head;
        while(temp->next != NULL)
        {
            temp = temp->next;
        }
        temp->next = p;
    }
}

void showList(Node *head)
{
    while(head != NULL)
    {
        cout << head->data << "  ";
        head = head->next;
    }
    cout << endl;
}

void swap(int& a, int& b)
{
    int tmp = b;
    b = a;
    a = tmp;
}

// 链表反转
Node *reverse(Node *head)
{
    Node *pCur = NULL;
    Node *pNext = NULL;
    
    if(head == NULL || head->next == NULL)
    {
        return head;
    }
    
    pCur = head->next;
    head->next = NULL;
    
    while(pCur != NULL)
    {
        pNext = pCur->next;
        pCur->next = head;
        head = pCur;
        pCur = pNext;
    }
    
    return head;
}

// 链表是否成环
bool isCirCle(Node *head)
{
    bool bIsCirCle = false;
    Node *pFast = NULL;
    Node *pSlow = NULL;
    
    if(head == NULL || head->next == NULL)
    {
        return bIsCirCle;
    }
    
    pFast = head;
    pSlow = head;
    while(pFast->next != NULL && pFast->next->next != NULL)
    {
        pFast = pFast->next->next;
        pSlow = pSlow->next;
        
        if(pFast == pSlow)
        {
            bIsCirCle = true;
            break;
        }
    }
    
    return bIsCirCle;
}

int main()
{
    Node *head = NULL;
    
    addNode(&head, createNode(2));
    addNode(&head, createNode(5));
    addNode(&head, createNode(7));
    addNode(&head, createNode(4));
    addNode(&head, createNode(6));
    addNode(&head, createNode(3));
    addNode(&head, createNode(1));
    addNode(&head, createNode(9));
    addNode(&head, createNode(8));

    cout << "Sort Before:" << endl;
    showList(head);
    
    //ListSort_1(&head);
    //ListSort_2(&head);
    ListSort_4(&head);
    
    cout << "Sort After:" << endl;
    showList(head);
    
    cout << "revert:" << endl;
    showList(reverse(head));
    
    cout << "list is circle: " << isCirCle(head) << endl;

    while(1);
    return 0;
}