141. Linked List Cycle && 142. Linked List Cycle II

141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null || head.next == null)
            return false;
        
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null)
        {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast)
                return true;
        }
        return false;
    }
}

 

142. Linked List Cycle II

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.

 

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head == null || head.next == null)
            return null;
        
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null)
        {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast)
                break;
        }
        if(fast.next == null || fast.next.next == null)
            return null;
        
        ListNode l = head;
        while(l != slow)
        {
            l = l.next;
            slow = slow.next;
        }
        return l;
    }
}