leetcode:141 环形链表

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    let p1=head;
    let p2=head;
    while(p1&&p2&&p2.next)
    {
        p1=p1.next;
        p2=p2.next.next;
        if(p1==p2)
        {
            return true;
        }
    }
    return false;
};

 let p1=head;
    let p2=head;

 p1=p1.next;
 p2=p2.next.next;
 为什么if(p1==p2)
        {
            return true;