判断链表是否有环

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define LEN 10

typedef struct node {
    int data;
    struct node *pnext;
} node_t, *pnode_t;

pnode_t create_list(int len)
{
    pnode_t phead, pnew, ptail;
    int i;

    for (i = 0; i < len; i++) {
        pnew = (pnode_t)malloc(sizeof(node_t));
        if (pnew == NULL) {
            printf("malloc fail.\n");
            exit(-1);
        }
        memset(pnew, 0, sizeof(node_t));

        pnew->data = i + 1;
        if (i == 0)
            phead = ptail = pnew;
        else {
            pnew->pnext = NULL;
            ptail->pnext = pnew;
            ptail = pnew;
        }

    }

    /*ptail->pnext = phead;*/
    return phead;
}

int main(int argc, char *argv[])
{
    pnode_t phead, p, q;

    phead = create_list(LEN);

    p = q = phead;
    while (1) {
        p = p->pnext;
        q = q->pnext->pnext;
        if (p == NULL || q == NULL) {
            printf("No Ring!\n");
            return;
        }
        if (p == q) {
            printf("Ring occurred!\n");
            return;
        }
    }

    return 0;
}