142环形链表II.md

1. 快慢指针，快指针走2，慢指针走1
2. 相遇后慢指针回到head，两个指针同时走1，再次相遇点则是环起点
3. 如没有相遇则最后返回null

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func detectCycle(head *ListNode) *ListNode {
    slow, fast := head, head
    for fast != nil && fast.Next != nil {
        slow = slow.Next
        fast = fast.Next.Next
        if slow == fast {
            slow = head
            for slow != fast {
                slow = slow.Next
                fast = fast.Next
            }
            return slow
        }
    }
    return nil
}