Question Definition
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
Note: Do not modify the linked list.
Follow up:
Can you solve it without using extra space?
Java Solution
public ListNode detectCycle(ListNode head) {
if(head == null || head.next == null) return null;
ListNode slow = head;
ListNode fast = head;
while(fast != null && fast.next != null){
slow = slow.next;
fast = fast.next.next;
if(slow == fast) break;
}
if(fast == null || fast.next == null) return null;
slow = head;
while(fast != slow){
slow = slow.next;
fast = fast.next;
}
return fast;
}
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