Detect Cycle in Linked List

Given a linked list, return true if there is a cycle in it. Otherwise, return false.

Difficulty:

JavaScript Solution

function hasCycle(head) {
  let slow = head, fast = head;
  while(fast && fast.next){
    slow = slow.next;
    fast = fast.next.next;
    if(slow === fast) return true;
  }
  return false;
}

Explanation: Use slow and fast pointers. If they meet, a cycle exists.

⏱ O(n) | 💾 O(1)

Java Solution

class Solution {
  public boolean hasCycle(ListNode head) {
    ListNode slow = head, fast = head;
    while(fast != null && fast.next != null){
      slow = slow.next;
      fast = fast.next.next;
      if(slow == fast) return true;
    }
    return false;
  }
}

Explanation: Floyd's cycle detection (tortoise and hare) efficiently detects cycles.

⏱ O(n) | 💾 O(1)

Python Solution

def has_cycle(head):
    slow, fast = head, head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            return True
    return False

Explanation: Floyd’s algorithm: slow and fast pointers detect cycle when they meet.

⏱ O(n) | 💾 O(1)

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Detect Cycle in Linked List