Leetcode 141 Linked List Cycle

题目：

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

Follow up:

Can you solve it without using extra space?

思路：

1）将遍历过的结点都指向头结点，遍历一个结点之前判断该结点的next的next的结点是否是头结点，如果是头结点的话，则存在环

2）设置两个指针，步数不同 如果存在环 则两个指针有指向同一结点的时候

思路1）代码

public boolean hasCycle(ListNode head) {
if(head == null){
return false;
}
ListNode phead = head;
ListNode p = head.next;
if(p == null){
return false;
}
while(p.next!=null){
if(phead.next.next == head){
return true;
}
phead = p.next;
p.next = head;
p = phead;
}
return false;
}

思路2代码

public boolean hasCycle(ListNode head) {
if(head == null){
return false;
}
ListNode phead = head;
ListNode p = head;
while(p.next!=null && p.next.next!=null){
phead = phead.next;
p = p.next.next;
if(phead == p){
return true;
}
}
return false;
}