142. Circular linked list II

Given the head node of a linked list head , Return to the first node of the link where the list begins to enter . If the list has no links , Then return to null.

If there is a node in the linked list , It can be done by continuously tracking next The pointer reaches again , Then there is a ring in the linked list . To represent a ring in a given list , The evaluation system uses an integer pos To indicate where the end of the list is connected to the list （ Index from 0 Start ）. If pos yes -1, There are no links in the list . Be careful ：pos Not passed as an argument , Just to identify the actual situation of the linked list .

No modification allowed Linked list .

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Ideas ： Speed pointer When they met, the quick pointer had gone n Double ring therefore Let's start again Step-by-step Meeting the slow pointer is the circular entrance