给定一个链表，如果链表中存在环，则返回到链表中环的起始节点的值，如果没有环，返回null。

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The node where the cycle begins. 
     *           if there is no cycle, return null
     */
    ListNode *detectCycle(ListNode *head) {
        // write your code here
        
        if(head == NULL || head->next == NULL) { return NULL; }
        
        ListNode * front = head;
        ListNode * tail = head;
        
        while(front && front->next) {
            front = front->next->next;
            tail = tail->next;
            
            if(front == tail) {
                break;
            }
        }
        
        if(front && front == tail) {
            front = head;
            while(front != tail && front != NULL && tail != NULL) {
                front = front->next;
                tail= tail->next;
            }
            
            return front;
        } else {
            return NULL;
        }
    }
};