142.cpp

// towpointer.cpp
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
  ListNode *detectCycle(ListNode *head) {
    ListNode *p1 = hasCycle(head), *p2 = head;
    if (p1 == nullptr)
      return nullptr;
    while (p1 != p2) {
      p1 = p1->next;
      p2 = p2->next;
    }
    return p1;
  }
  ListNode *hasCycle(ListNode *head) {
    ListNode *fast = head, *slow = head;
    while (fast && fast->next) {
      fast = fast->next->next;
      slow = slow->next;
      if (fast == slow)
        return fast;
    }
    return nullptr;
  }
};