环形链表 II

• 直接遍历链表，使用set做标记位（标记是否已经到达过）

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
type void struct{}
func detectCycle(head *ListNode) *ListNode {
    if head==nil||head.Next==nil{
        return nil
    }
    
    var nodes = map[*ListNode]void{}
    for head!=nil{
        if _, ok := nodes[head];ok{
            return head
        }
        nodes[head] = void{}
        head = head.Next
    }
    
    return nil
}

• 快慢指针
  • 先用快慢指针判断是否存在环，
  • 然后将满指针移动到head，
  • 然后以相同的速度同时移动快慢指针，相遇点即为环入口

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func detectCycle(head *ListNode) *ListNode {
    if head==nil||head.Next==nil{
        return nil
    }

    var flag = false//是否存在环
    var slow = head
    var quick = head

    for quick!=nil && quick.Next!=nil{
        slow = slow.Next
        quick = quick.Next.Next
        if quick==slow{
            flag = true
            break
        }     
    }

    if flag{
        slow = head
        for slow!=quick{
            slow = slow.Next
            quick = quick.Next
        }
        return slow
    }

    return nil

}