Linked list cycle ii

Hash Table Linked List Two Pointers

57.3% acceptance

Jan 13, 2026

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1085

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

Solution

C++

Time O(n²)

Space O(1)

LeetCode

solution.cpp

class Solution {
public:
  ListNode *detectCycle(ListNode *head) {
    ListNode *slow = head, *fast = head;
    while (fast && fast->next) {
      slow = slow->next;
      fast = fast->next->next;
      if (slow == fast) {
        slow = head;
        while (slow != fast) {
          slow = slow->next;
          fast = fast->next;
        }
        return slow;
      }
    }
    return nullptr;
  }
};