Detect a Cycle in a Linked List

📌 Definition:

📖 Detailed Explanation:

Floyd's cycle detection uses slow/fast pointers: if there is a cycle, the fast pointer eventually laps and meets the slow one; if fast reaches null, there is no cycle. O(n) time, O(1) space — no extra visited set needed.

💻 Solutions:

• Java
• JavaScript
• Python

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

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;
}

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

🔑 Key Points:

• Floyd's slow/fast pointers
• They meet inside a cycle; fast hits null otherwise
• O(1) space vs a visited hash set
• Extendable to find the cycle start

🌍 Real-World Example: