题解 | #判断链表中是否有环#

 function ListNode(x){
  this.val = x;
  this.next = null;
} 
function hasCycle( head ) { 
    if (head === null || head.next === null) return false; 
    
    let fast = head.next;
    let slow = head;
    
    while (fast && fast.next) {
        if (fast === slow) return true;
        fast = fast.next.next;
        slow = slow.next;
    }
    return false;
}
module.exports = {
    hasCycle : hasCycle
};