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?

快慢指针，没啥好说的了。

--------------------------------

/**
 * 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) {
        ListNode *lo = head, *hi = head;
        while (hi != NULL && hi->next != NULL) {
            hi = hi->next->next;
            lo = lo->next;
            if (hi == lo) return true;
        }
        
        return false;
    }
};