Judge whether a single-chain table has a ring. If yes, search for the entry point of the ring and use the speed pointer method.

Pnode findbeginning (pnode head) {pnode p1 = head; pnode P2 = head; // Step 1: Determine whether the linked list has a ring while (P2-> next! = NULL) {p1 = p1-> next; P2 = P2-> next; If (p1 = P2) // The linked list does have a loop break ;} if (P2-> next = NULL) // return NULL, which does not exist in the linked list; // Step 2: start searching for the entry point P1 = head of the ring; // Let P1 return to the start point of the linked list, and P2 stay at the encounter point while (P1! = P2) {// when P1 and P2 meet again, that point is the ring's entry point P1 = p1-> next; P2 = P2-> next ;} return P2 ;}

Judge whether a single-chain table has a ring. If yes, search for the entry point of the ring and use the speed pointer method.