环形链表

**141. Linked List Cycle **
Given a linked list, determine if it has a cycle in it.
代码如下：

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while(slow&&fast&&fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if(fast==slow)
                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.
代码如下：

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while(slow&&fast&&fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if(slow==fast)
                break;
        }
        if(fast==NULL||fast->next==NULL)
           return NULL;
        slow = head;
        while(fast!=slow)
        {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};