매일코딩 4월 16일 문제: http://oj.leetcode.com/problems/linked-list-cycle/

solution.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
 #include <map>
 
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == NULL) return false;
        
        std::map<ListNode*, bool> table;
        
        while (!table[head]) {
            table[head] = true;
            if (head->next == NULL) {
                return false;
            }
        
            head = head->next;
        }
        
        return true;
    }
};

solution.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) {
        if (head == null) return false;
        
        Map<ListNode, Boolean> table = new HashMap<ListNode, Boolean>();
        while (table.get(head) == null) {
            table.put(head, true);
            if (head.next == null) {
                return false;
            }
            head = head.next;
        }
        return true;
    }
}

solution.py

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

class Solution:
    # @param head, a ListNode
    # @return a boolean
    def hasCycle(self, head):
        if head == None:
            return False
        
        table = dict();
        while head not in table:
            table[head] = True
            if head.next == None:
                return False
            head = head.next
        return True