142. 环形链表 II

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方案一: 判圈算法

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;
};