142. Linked List Cycle II

双指针，找到重合点，然后新一个指针从头出发，两个指针交汇的地方就是圈的起点

    public ListNode detectCycle(ListNode head) {
        if(head == null) {
            return null;
        }
        ListNode runner = head;
        ListNode walker = head;
        boolean hasCycle = false;
        while(runner.next != null && runner.next.next != null) {
            walker = walker.next;
            runner = runner.next.next;
            if(walker == runner) {
                hasCycle = true;
                break;
            }
        }
        ListNode res = head;
        if(hasCycle) {
            while(walker != res) {
                res = res.next;
                walker = walker.next;
            }
            return res;
        }
        return null;
    }