Linked List Cycle II

Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Follow up:
Can you solve it without using extra space?

#include
#include

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

ListNode* detectCycle(ListNode *head) {
    ListNode *p1,*p2,*p3;
    int n,m,flag;
    if(head==NULL||head->next==NULL) return NULL;
    if(head->next==head) return head;
    for(p1=p2=head;p1!=NULL&&p1->next!=NULL;){
        p1=p1->next->next;
        p2=p2->next;
        if(p1==p2) break;
    }
    if(p1==NULL || p1->next==NULL) return NULL;
    n=1;
    while(p1->next!=p2){
        p1=p1->next;
        n++;
    }
    for(p1=head; p1!=p2; p1=p1->next){
        for(p3=p1, m=0, flag=0; p3->next!=p1 && m<=n; p3=p3->next){
            if(p3==p2) flag=1;
            if(flag==1) m++;
        }
        if(p3->next==p1) return p1;
    }
    return p2;
}