链表 - 环形链表

142. 环形链表

方法一：哈希表

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
    const visited = new Set();
    while (head !== null) {
        if (visited.has(head)) {
            return head;
        }
        visited.add(head);
        head = head.next;
    }
    return null;
};

方法二：快慢指针

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
    if (head === null) {
        return null;
    }
    let slow = head, fast = head;
    while (fast !== null) {
        slow = slow.next;
        if (fast.next !== null) {
            fast = fast.next.next;
        } else {
            return null;
        }
        if (fast === slow) {
            let ptr = head;
            while (ptr !== slow) {
                ptr = ptr.next;
                slow = slow.next;
            }
            return ptr;
        }
    }
    return null;
};