数据结构——单链表（环）

一、环

标题

 

二、创建环

//建立环
    public void createLoop(){
        Entry cur = this.head;
        while(cur.next != null){
            cur = cur.next;
        }
        Entry cur2 = this.head.next.next.next;
        cur.next = cur2;//和第三个数据结点构成一个环
    }

三、判断是否有环

//判断一个单链表中是否有环
    public boolean isLoop(){
        //快指针和慢指针从head出发
        Entry fast = this.head;
        Entry slow = this.head;
        while(fast != null && fast.next != null){
            //fast每走两步，slow走一步
            fast = fast.next.next;
            slow = slow.next;
            if(slow == fast){//有环的话,fast和slow迟早会相遇
                return true;
            }
        }
        //没有相遇就没有环
        return false;
    }

四、换的入口点

    //环的入口点
    public int enterEntry(){
        Entry fast = this.head;
        Entry slow = this.head;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(slow == fast){
                break;
            }
        }
        slow = this.head;
        while(fast != slow){
            fast = fast.next;
            slow = slow.next;
        }
        return slow.data;
    }

五、环的长度

    //环的长度
    public int loopLength(){
        Entry fast = this.head;
        Entry slow = this.head;
        boolean flag = false;//标记
        int len = 0;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(slow == fast && flag == true){
                break;
            }

            if(slow == fast && flag == false){
                flag = true;//第一次相遇将flag置为true
            }
            if(flag == true){
                len ++;
            }
        }
        return len;
    }