判断链表是否有环

快慢指针法

function hasCircle(head) {
    let fast = head;
    let slow = head;
    while (fast && slow && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
        if (fast === slow) {
            return true;
        }
    }
    return false;
}

记录法

function hasCircle(head) {
    let visitedSet = new Set();
    while (head) {
        if (visitedSet.has(head)) {
            return true;
        }
        visitedSet.add(head);
        head = head.next;
    }
    return false;
}