Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

题意：给定一个链表，判断是否有环

由于这题出现的频率太高，我也不小心很早就看过它的做法，SO，直接贴代码吧！

PS：对于链表关键要考虑它的健壮性，异常情况有：空链表、单节点、空指针的情况

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

bool hasCycle(ListNode *head) {
bool flag = false;
if(head != NULL){
ListNode *p1 = head;
ListNode *p2 = head;
int i = 0;
while(p1->next){
i++;
p1 = p1->next;
if( i%2 == 0) p2 = p2->next;
if(p1 == p2){
flag = true;
break;
}
}
}
return flag;
}