141. Linked List Cycle

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

Follow up: Can you solve it without using extra space?

Thoughts:

1. slow and fast pointer

2. Change the value of the node (mark)

Code 1

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head) return false;
        ListNode* slow = head, *fast = head;
        while(fast->next && fast->next->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast) return true;
        }

        return false;
    }
};

Code 1 Using Exception (Python)

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        try:    
            slow = head
            fast = head.next
            while slow is not fast:
                slow = slow.next
                fast = fast.next.next
            return True;

        except:
            return False;

Code 2 (Java)

/**
 * 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) {
    return head!=null && (head.val == (head.val = 0xcafebabe) || hasCycle(head.next));
    }
}