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142. Linked List Cycle II 👍

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  ListNode* detectCycle(ListNode* head) {
    ListNode* slow = head;
    ListNode* fast = head;

    while (fast != nullptr && fast->next != nullptr) {
      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;
  }
};

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public class Solution {
  public ListNode detectCycle(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;

    while (fast != null && fast.next != null) {
      slow = slow.next;
      fast = fast.next.next;
      if (slow == fast) {
        slow = head;
        while (slow != fast) {
          slow = slow.next;
          fast = fast.next;
        }
        return slow;
      }
    }

    return null;
  }
}

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class Solution:
  def detectCycle(self, head: ListNode) -> ListNode:
    slow = head
    fast = head

    while fast and 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 None