快慢指针

//最快的方式查找一个链表中的最中间节点
#include<stdio.h>
#define SIZE 100000
typedef struct Node{
    int data;
    struct Node* next;
    struct Node* prev;
}Node;
Node f[SIZE];
int index = 0;
Node* head;
Node* getnode(){
    return &f[index++];
}
void init(){
    index = 0;
    head = getnode();
    head->prev = head;
    head->next = head;
    head->data = -1;
}
void InsertAfter(Node*dst,Node*newnode){
    newnode->prev = dst;
    newnode->next = dst->next;
    dst->next->prev = newnode;
    dst->next = newnode;
}

int main(){
    init();
    Node* temp = getnode();//head的地址要保存下来用于遍历
    for(int i=1;i<21;i++){
        Node* p = getnode();
        p->data = i;
        if(i==1){temp = head;}
        else{temp=temp->next;}
            InsertAfter(temp,p);
    }
    Node* p1,*p2;//快慢指针
    p1 = head->next;
    p2 = head->next;
    while(p1->data!=-1){
        p1 = p1->next->next;
        p2 = p2->next;
    }
    printf("%d\n",p2->prev->data);
    return 0;
}