142 Linked List Cycle II

Leetcode

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.

Do not modify the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.

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和 Linked List Cycle 有些像，也是通过一快一慢两个指针判断链表是否又环。有一个巧妙的方法找到入口节点。两个指针相遇后，一个指针从相遇的时候开始，一个指针从头开始，以相同速度前进，相遇的节点为入口节点。

证明：定义从 head 到入口距离为 a ，从入口到相遇处距离为 b ，从相遇处继续前进到入口距离为 c 。环长度 l = b + c 。有 a+2b+c+nl=2a+2b （n为正整数，代表转到圈数） ，则 nl+c=a ，所以，两个指针会在入口处相遇。

public ListNode detectCycle(ListNode head) {
    ListNode fast=head, slow=head;
    do{
        if(fast!=null && fast.next!=null) {
            fast=fast.next.next;
            slow=slow.next;
        } else {
            return null;
        }
    } while(fast!=slow);
    slow=head;
    while(fast!=slow){
        fast=fast.next;
        slow=slow.next;
    }
    return slow;
}