【leetcode打卡】环形链表 II

环形链表 II

环形链表 II

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        //两个问题，判断是否有环
        //如果有环，怎样找到环的入口

        //利用双指针找环
         ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null){
             slow = slow.next;
            fast = fast.next.next;
            //快指针跳2步，慢指针跳1步，如果有环的话，两指针一定会相遇
            if(slow==fast){
                //推理得公式x=nz+(n-1)y;x:起点到入环结点，y:入换节点到相遇结点，z:相遇结点到入环结点
                ListNode index1 = fast;
                ListNode index2 = head;
                while (index1 != index2) {
                    index1 = index1.next;
                    index2 = index2.next;
                }
                return index1;
            }
        }
        return null;
    }
}