Linked List Cycle

Easy

Python

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

Java

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;
}

Key Insight

Floyd's cycle. Python truthiness vs Java null checks.

Python → Java Differences

while fast and fast.next vs !=null
True/False vs true/false
Floyd's identical

Python

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

Java

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;
}

Algorithm Steps

1. Slow and fast pointers
2. Meet: cycle

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